[golang14] fix TestParseInLocation test: Install package from latest changes on upstream go14 branch

https://github.com/golang/go/commit/4d5426a570c2820c5894a61b52e3dc147e4e7925

golang14 is still failing to build on some server, the commit above fix the
problem on test TestParseInLocation. This is why we install golang14 from upstream go14 branch.

[ kirr: see also relateddiscussion on nexedi/slapos!133 ]

/reviewed-by @kirr
/reviewed-on nexedi/slapos!245