Misc: allow unknown args when finding prefix #1618

Job	Run time
linux-newLTS-vanilla--dotnet6	9m 48s
linux-newLTS-vanilla--newmono	8m 35s
linux-newLTS-github--dotnet-and-xbuild	8m 18s
linux-newLTS-github--dotnet-and-msbuild	8m 0s
linux-oldLTS-github--dotnet-and-msbuild	8m 50s
linux-oldLTS-github--dotnet-and-xbuild	7m 25s
linux-newLTS-vanilla--stockmono	6m 21s
linux-oldLTS-vanilla--newmono	8m 30s
linux-oldLTS-vanilla--stockmono	5m 50s
macOS--dotnet6-and-mono	7m 54s
windows--dotnet6	10m 13s
macOS--mono	7m 28s
windows--legacyFramework	13m 44s
sanity-check	1m 13s
publish	1m 1s
	1h 53m 10s

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