Mtime of source codes are sometimes newer than build time (clock skew?)

This clock skew causes make install failed with Permission denied.
It should not be happen when mtime of source codes are sane.

https://travis-ci.org/ruby/ruby/jobs/525784924

before_script.1:

$ date
Mon Apr 29 03:18:32 GMT 2019

before_script.32:

++date
Mon Apr 29 02:55:14 GMT 2019

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