update TCTI environment variable usage

Keylime Rust agent does not understand TPM2TOOLS_ prefixed
configuration. For example, TPM2TOOLS_TCTI is ignored when setting up
swtpm.

.ci/test_wrapper.sh is already exporting both formats, but
installation documentation is not mentioning the shorter format at all.

Signed-off-by: Tuomo Tanskanen <tuomo.tanskanen@est.tech>

docs/installation.rst

@@ -160,17 +160,20 @@ the :code:`tpm2_checkquote` utility in your path.
     resource manager (available `here <https://github.com/tpm2-software/tpm2-abrmd>`_).
 
 How the TPM is accessed by tpm2-tools can be set using the :code:`TPM2TOOLS_TCTI` environment
-variable. More information about that can be found
+variable. Keylime Rust agent understands :code:`TCTI` without the prefix.
+More information about that can be found
 `here <https://github.com/tpm2-software/tpm2-tools/blob/master/man/common/tcti.md>`_.
 
 Talk to the swtpm emulator directly::
 
     export TPM2TOOLS_TCTI="mssim:port=2321"
+    export TCTI="mssim:port=2321"
 
 
 To talk to the TPM directly (not recommended)::
 
     export TPM2TOOLS_TCTI="device:/dev/tpm0"
+    export TCTI="device:/dev/tpm0"
 
 
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