Add support for BRIEFCASE_DEBUG. #92

Job	Run time
Pre-commit checks / Pre-commit Checks	20s
Build apps (3.10, pygame) / Verify App Build	2m 42s
Build apps (3.10, toga) / Verify App Build	3m 50s
Build apps (3.10, ppb) / Verify App Build	3m 20s
Build apps (3.10, pyside6) / Verify App Build	4m 22s
Build apps (3.11, ppb) / Verify App Build	3m 33s
Build apps (3.11, pygame) / Verify App Build	3m 17s
Build apps (3.11, toga) / Verify App Build	3m 11s
Build apps (3.11, pyside6) / Verify App Build	4m 45s
Build apps (3.12, ppb) / Verify App Build	3m 54s
Build apps (3.12, pygame) / Verify App Build	3m 3s
Build apps (3.12, pyside6) / Verify App Build	5m 5s
Build apps (3.12, toga) / Verify App Build	3m 39s
Build apps (3.8, ppb) / Verify App Build	3m 20s
Build apps (3.8, pygame) / Verify App Build	2m 41s
Build apps (3.8, pyside6) / Verify App Build	4m 37s
Build apps (3.8, toga) / Verify App Build	3m 27s
Build apps (3.9, ppb) / Verify App Build	4m 2s
Build apps (3.9, pygame) / Verify App Build	3m 29s
Build apps (3.9, pyside6) / Verify App Build	4m 52s
Build apps (3.9, toga) / Verify App Build	3m 10s
	1h 14m 39s

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