Some small cmake tweaks

[ChrisCummins]

Some small tweaks to attempt to reduce the number of flags needed to pass to cmake for most use cases:

• Python3_FIND_VIRTUALENV=FIRST is the default value, so don't specify it in install instructions.
• Define CMAKE_BUILD_WITH_INSTALL_RPATH in CMakeLists.txt rather than from command line.
• Don't override command line flags to set ccache / lld paths.

[sogartar]

I don't think that the CompilerGym Python package utilizes rpath. It should also be installable anywhere. We don't install through CMake's install target. CMAKE_BUILD_WITH_INSTALL_RPATH will hardcode the rpath based on CMAKE_INSTALL_PREFIX.

[ChrisCummins]

Is it a good idea to hardcode it as I have done here?

[ChrisCummins]

Deferring to you on this @sogartar as the cmake expert 🙂

[sogartar]

I think we don't need CMAKE_BUILD_WITH_INSTALL_RPATH because the service executables link statically to their dependencies.

[codecov-commenter]

Codecov Report

Merging #666 (e43ad7b) into development (63dbfac) will increase coverage by 0.03%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           development     #666      +/-   ##
===============================================
+ Coverage        88.36%   88.40%   +0.03%     
===============================================
  Files              125      125              
  Lines             7565     7565              
===============================================
+ Hits              6685     6688       +3     
+ Misses             880      877       -3     

Impacted Files	Coverage Δ
compiler_gym/envs/llvm/datasets/cbench.py	80.50% <0.00%> (+1.08%)	⬆️

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