Add support for configuring `xwin` using env vars #3822

Job	Run time
Generate Matrix	5s
Test Alpine Linux	0s
Test Auditwheel	0s
Test Cross Compile	0s
Test Docker	0s
Test Bootstrap (ubuntu-latest)	6s
Test Emscripten	0s
Test Bootstrap (macos-14)	7s
Test MSRV	2m 20s
Test Bootstrap (windows-latest)	15s
Test Pyston	0s
Check ${{ matrix.platform.target }}	0s
Test downstream - ${{ matrix.downstream.repository }}	0s
Test (ubuntu-latest, 3.12)	28m 25s
Test (ubuntu-latest, pypy3.10)	11m 36s
Test (macos-14, 3.12)	8m 40s
Test (windows-latest, 3.12)	20m 46s
Test (windows-latest, pypy3.10)	20m 37s
conclusion	1s
	1h 32m 58s

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