Stop using dune subst?

[OlivierNicole]

The Dune build of JSOO and WSOO relies on dune subst (the default, IIUC). It has the unfortunate consequence that it’s not really possible to build from a source tree that is not a git repository, which has bitten me on two occasions:

• once when trying to pin wasm_of_ocaml-compiler, and
• once when setting up a benchmarking CI (Add wasm_of_ocaml benchmarks with current-bench output #1842). To work around it, I used the dirty hack of setting up a fake Git repository.

Is dune subst useful enough to justify these pain points?

[OlivierNicole]

Thanks!