Tests: Teach RunCMake.SymlinkTrees to tolerate CCACHE_BASEDIR

Brad Kingrequested to merge
brad.king/cmake:test-symlinks-with-ccache into master

If the test is driven with a ccache-wrapped compiler then the CCACHE_BASEDIR environment variable might break paths the test checks.

Fixes: #23885 (closed)
Backport: release

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