ci: avoid collecting coverage data on local dev

[Oppen]

Add the TEST_COLLECT_COVERAGE environmental variable to the Makefile
and use it to determine whether to use llvm-cov when running tests via
the make target.
Set it for CI so the pipelines still collect it. This should both make
local dev faster and save our disk space.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e0f37ad) 97.17% compared to head (b3f2ecd) 97.17%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1519   +/-   ##
=======================================
  Coverage   97.17%   97.17%           
=======================================
  Files          91       91           
  Lines       37175    37175           
=======================================
  Hits        36124    36124           
  Misses       1051     1051           

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[github-actions]

Benchmark Results for unmodified programs 🚀

Command	Mean [s]	Min [s]	Max [s]	Relative
base big_factorial	2.255 ± 0.017	2.236	2.287	1.00
head big_factorial	2.267 ± 0.021	2.243	2.303	1.01 ± 0.01

Command	Mean [s]	Min [s]	Max [s]	Relative
base big_fibonacci	2.238 ± 0.015	2.227	2.271	1.00
head big_fibonacci	2.243 ± 0.019	2.221	2.275	1.00 ± 0.01

Command	Mean [s]	Min [s]	Max [s]	Relative
base blake2s_integration_benchmark	8.247 ± 0.091	8.142	8.340	1.00
head blake2s_integration_benchmark	8.271 ± 0.162	8.137	8.685	1.00 ± 0.02

Command	Mean [s]	Min [s]	Max [s]	Relative
base compare_arrays_200000	2.302 ± 0.013	2.284	2.315	1.00
head compare_arrays_200000	2.313 ± 0.017	2.295	2.335	1.00 ± 0.01

Command	Mean [s]	Min [s]	Max [s]	Relative
base dict_integration_benchmark	1.441 ± 0.012	1.432	1.466	1.00 ± 0.01
head dict_integration_benchmark	1.440 ± 0.007	1.433	1.457	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base field_arithmetic_get_square_benchmark	1.299 ± 0.007	1.290	1.313	1.00 ± 0.01
head field_arithmetic_get_square_benchmark	1.296 ± 0.010	1.285	1.312	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base integration_builtins	8.255 ± 0.092	8.146	8.365	1.00 ± 0.02
head integration_builtins	8.241 ± 0.110	8.180	8.538	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base keccak_integration_benchmark	8.504 ± 0.090	8.406	8.616	1.01 ± 0.01
head keccak_integration_benchmark	8.428 ± 0.034	8.374	8.473	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base linear_search	2.320 ± 0.017	2.296	2.357	1.00
head linear_search	2.355 ± 0.037	2.333	2.458	1.02 ± 0.02

Command	Mean [s]	Min [s]	Max [s]	Relative
base math_cmp_and_pow_integration_benchmark	1.591 ± 0.018	1.575	1.637	1.00 ± 0.01
head math_cmp_and_pow_integration_benchmark	1.589 ± 0.005	1.581	1.598	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base math_integration_benchmark	1.459 ± 0.006	1.451	1.471	1.00
head math_integration_benchmark	1.463 ± 0.008	1.458	1.485	1.00 ± 0.01

Command	Mean [s]	Min [s]	Max [s]	Relative
base memory_integration_benchmark	1.271 ± 0.010	1.258	1.287	1.00
head memory_integration_benchmark	1.289 ± 0.012	1.273	1.308	1.01 ± 0.01

Command	Mean [s]	Min [s]	Max [s]	Relative
base operations_with_data_structures_benchmarks	1.625 ± 0.009	1.615	1.648	1.00 ± 0.01
head operations_with_data_structures_benchmarks	1.625 ± 0.010	1.617	1.646	1.00

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base pedersen	606.2 ± 2.8	602.9	610.7	1.00
head pedersen	607.8 ± 4.0	603.1	615.3	1.00 ± 0.01

Command	Mean [s]	Min [s]	Max [s]	Relative
base poseidon_integration_benchmark	1.066 ± 0.005	1.060	1.074	1.00
head poseidon_integration_benchmark	1.068 ± 0.002	1.062	1.070	1.00 ± 0.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base secp_integration_benchmark	1.932 ± 0.003	1.926	1.937	1.00
head secp_integration_benchmark	1.944 ± 0.011	1.933	1.973	1.01 ± 0.01

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base set_integration_benchmark	772.8 ± 1.8	771.0	776.8	1.00 ± 0.00
head set_integration_benchmark	772.5 ± 2.9	769.7	779.0	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base uint256_integration_benchmark	4.591 ± 0.036	4.549	4.645	1.01 ± 0.01
head uint256_integration_benchmark	4.546 ± 0.016	4.525	4.573	1.00