Look in current directory for resources directory too

[5225225]

This change scans for the resources directory both from the env::current_exe() directory, as well as the current directory, so if your exe is outside of the servo repo, it can still be found.

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #___ (GitHub issue number if applicable)

• There are tests for these changes OR
• These changes do not require tests because ___

[5225225]

I had a quick look around, and couldn't see any tests that would have called this. Unclear to me exactly how you'd test this.