test.yml: stop testing tailscale repo (effectively a moving target due #119

Job	Run time
test-external-repos (terraform_chain)	2m 35s
test-gotip (ubuntu-latest)	23s
test (1.21.x, ubuntu-latest)	45s
test-gotip (macos-latest)	52s
test (1.21.x, macos-latest)	1m 19s
test-external-repos (terraform_nochain)	2m 41s
test-gotip (windows-latest)	1m 32s
test (1.21.x, windows-latest)	3m 10s
test (1.20.x, ubuntu-latest)	58s
test (1.20.x, macos-latest)	1m 12s
test (1.20.x, windows-latest)	3m 15s
	18m 42s

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