Add warning for when git fails in build.rs

[SethDusek]

Hi, it seems the coveralls pipeline has been failing in build.rs (git parse --show-cdup fails), which causes the program to fail to compile due to git env vars not being defined.

This PR doesn't really address the issue directly, but instead it warns of the failed command in build output, and replaces the env vars with blanks. Here's the error message from git itself inside the pipeline:

stderr = "fatal: detected dubious ownership in repository at '/w/oracle-core/oracle-core'\nTo add an exception for this directory, call:\n\n\tgit config --global --add safe.directory /w/oracle-core/oracle-core\n"

[coveralls]

Pull Request Test Coverage Report for Build 4125457486

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 61.491%

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Totals
Change from base Build 4052230392:	0.0%
Covered Lines:	2012
Relevant Lines:	3272

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💛 - Coveralls