internal/coordinator/remote: use correct syntax to expand PATH on Win…

…dows

For golang/go#56015.

Change-Id: Ic16f9b8e995424f50fda6921228a75683f91f8ff
Reviewed-on: https://go-review.googlesource.com/c/build/+/437817
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

internal/coordinator/remote/ssh.go

@@ -320,7 +320,7 @@ func (ss *SSHServer) setupRemoteSSHEnv(bconf *dashboard.BuildConfig, workDir str
 			fmt.Fprintf(f, "set %s\n", env)
 		}
 		fmt.Fprintf(f, `set GOPATH=%s\gopath`+"\n", workDir)
-		fmt.Fprintf(f, `set PATH=$PATH;%s\go\bin`+"\n", workDir)
+		fmt.Fprintf(f, `set PATH=%%PATH%%;%s\go\bin`+"\n", workDir)
 		fmt.Fprintf(f, `cd %s\go\src`+"\n", workDir)
 	case "plan9":
 		// TODO