Switched to cc pattern in compiler detection

[hansenms]

Changelog: Fix: Fix gcc detection in conda environments.
Docs: Omit

In a conda environment, the CC and CXX variables point to /opt/conda/envs//bin/x86_64-conda-linux-gnu-cc and /opt/conda/envs//bin/x86_64-conda-linux-gnu-c++ and the command searched for pattern ends up being /opt/conda/envs//bin/x86_64-conda-linux-gnu-cc and thus _gcc_compiler(command) is never called.

Fixes #14575

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[memsharded]

Thanks very much for the contribution @hansenms

Please have a look to the comment. 2.1 is about to be branched, if ready it could be in, otherwise might need to wait until next release.

[memsharded]

Thanks for the contribution and the review!