cicd: don't publish preview build on external branch #93

Job	Run time
Test documentation build	44s
Lint & Test (macos-12)	1m 47s
Test Python package build (macos-12)	34s
Test Briefcase build (macos-12)	1m 48s
Lint & Test (windows-2022)	1m 39s
Test Python package build (windows-2022)	1m 11s
Test Briefcase build (windows-2022)	1m 45s
Lint & Test (ubuntu-22.04)	1m 36s
Test Python package build (ubuntu-22.04)	18s
Test Briefcase build (ubuntu-22.04)	1m 58s
CodeQL (javascript)	1m 13s
CodeQL (python)	1m 49s
Upload coverage	1s
Deploy github pages	1s
Briefcase build & draft release	1s
Publish to PyPi	1s
	16m 26s

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