make.fsx: DRY wrt getting proj file path #3207

Job	Run time
macOS--mono-only	10m 28s
macOS--mono-only	10m 28s
windows--dotnet6-and-legacyFramework	6m 11s
windows--dotnet6-and-legacyFramework	6m 11s
macOS--dotnet6-and-mono	14m 16s
macOS--dotnet6-and-mono	14m 16s
macOS--dotnet6-only	6m 55s
macOS--dotnet6-only	6m 55s
linux-github--dotnet-and-mono	3m 39s
linux-github--dotnet-and-mono	3m 39s
linux-vanilla--stockmono-only	4m 16s
linux-vanilla--stockmono-only	5m 2s
linux-github--dotnet-and-newmono	6m 37s
linux-github--dotnet-and-newmono	6m 37s
windows--dotnet6-only	6m 15s
windows--dotnet6-only	6m 15s
windows--legacyFramework-only	14m 29s
windows--legacyFramework-only	14m 29s
linux-vanilla--newmono-only	8m 29s
linux-vanilla--newmono-only	8m 29s
linux-vanilla--stockdotnet6-and-newmono	5m 35s
linux-vanilla--stockdotnet6-and-newmono	5m 35s
linux-vanilla--stockdotnet6-and-stockmono	7m 7s
linux-vanilla--stockdotnet6-and-stockmono	7m 7s
linux-vanilla--stockdotnet6-only	4m 59s
linux-vanilla--stockdotnet6-only	4m 59s
conventions	0s
conventions	1m 32s
snap_pkg	0s
snap_pkg	6m 52s
snap_pkg_beta	0s
snap_pkg_beta	7m 16s
	3h 34m 58s

Provide feedback