Allow setting configuration file location with environment variable

[nedbat]

Originally reported by Anonymous

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If i have a non-default configuration file location i end up with --rcfile=... arguments all over my tox.ini. i would really appreciate being able to setenv = COVERAGE_RCFILE=... in one place instead.

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• Bitbucket: https://bitbucket.org/ned/coveragepy/issue/650

[nedbat]

Implemented in 1d72d26cccd4 (bb)