cmd/go: avoid copying a binary to be exec'd in TestScript/gotoolchain_path

Runinng 'go build' writes the binary in a separate process, so avoids
the race described in #22315. However, the script engine's 'cp'
command currently executes in-process, so it does not avoid that bug
and may retain stale file descriptors when running tests in parallel.

Avoid the race in this particular test by giving the final binary
location in the '-o' argument instead of copying it there after the
fact.

Fixes #64019.

Change-Id: I96d276f33c09e39f465e9877356f1d8f2ae55062
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/560415
Auto-Submit: Bryan Mills <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>

Author: Bryan C. Mills

src/cmd/go/testdata/script/gotoolchain_path.txt

@@ -8,8 +8,7 @@ env TESTGO_VERSION=go1.21pre3
 # Compile a fake toolchain to put in the path under various names.
 env GOTOOLCHAIN=
 mkdir $WORK/bin
-go build -o $WORK/bin/ ./fakego.go  # adds .exe extension implicitly on Windows
-cp $WORK/bin/fakego$GOEXE $WORK/bin/go1.50.0$GOEXE
+go build -o $WORK/bin/go1.50.0$GOEXE ./fakego.go  # adds .exe extension implicitly on Windows
 
 [!GOOS:plan9] env PATH=$WORK/bin
 [GOOS:plan9] env path=$WORK/bin