x/build: spurious "=C:=C:\golang" in environment

[bcmills]

I noticed in https://storage.googleapis.com/go-build-log/18934e11/windows-amd64-2016_977b0eeb.log (a TryBot run on Windows) that the Windows builder environment includes a bogus entry that begins "=C:=C:\golang":

:: Running C:\workdir\go\bin\go with args […] and env ["GOBUILDEXIT=1" "=C:=C:\\golang" "ALLUSERSPROFILE=C:\\ProgramData" […]

This appears to be a very old bug: the earliest occurrence of it I can find dates to 2018:

2018-12-19T22:59:49-e147a91-f500f13/windows-amd64-2016

But even that may just be a reflection of the extent of my fetchlogs database rather than the extent of the bug.

[mknyszek]

CC @dmitshur @cagedmantis

[gopherbot]

Change https://golang.org/cl/367896 mentions this issue: cmd/coordinator: join paths using the host's path separator

[bcmills]

According to https://devblogs.microsoft.com/oldnewthing/20100506-00/?p=14133, the =C: variable is produced by cmd.exe and is basically benign. (It looks weird in the logs, but shouldn't cause any harm.)