cicd: don't publish preview build on external branch #92

Job	Run time
Test documentation build	43s
Lint & Test (macos-12)	3m 54s
Test Python package build (macos-12)	34s
Lint & Test (windows-2022)	3m 39s
Test Briefcase build (macos-12)	3m 5s
Test Python package build (windows-2022)	49s
Test Briefcase build (windows-2022)	4m 26s
Lint & Test (ubuntu-22.04)	1m 36s
Test Python package build (ubuntu-22.04)	26s
Test Briefcase build (ubuntu-22.04)	3m 11s
CodeQL (javascript)	1m 22s
CodeQL (python)	2m 10s
Upload coverage	2s
Deploy github pages	0s
Briefcase build & draft release	0s
Publish to PyPi	0s
	25m 57s

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