Set GOTMPDIR to a child directory (#4183)

So, Windows does not detect temporary Go binaries (e.g. magefile) as a
virus.

This solves:

```
failed to run compiled magefile: fork/exec C:\Users\windows\magefile\6e561802bbe21387b05a72ec1f51afbf0b1d8e8d.exe: Operation did not complete successfully because the file contains a virus or potentially unwanted software.
```

Author: rdner

.buildkite/hooks/pre-command.ps1

@@ -14,8 +14,12 @@ go version
 
 $GOPATH = $(go env GOPATH)
 $env:Path = "$GOPATH\bin;" + $env:Path
+$env:GOTMPDIR = "$GOPATH\tmp"
+New-Item -ItemType Directory -Force -Path $env:GOTMPDIR
 [Environment]::SetEnvironmentVariable("GOPATH", "$GOPATH", [EnvironmentVariableTarget]::Machine)
+[Environment]::SetEnvironmentVariable("GOTMPDIR", "$GOPATH\tmp", [EnvironmentVariableTarget]::Machine)
 [Environment]::SetEnvironmentVariable("Path", "$GOPATH\bin;$env:Path", [EnvironmentVariableTarget]::Machine)
+go env
 
 # Install tools
 go install github.com/magefile/mage