[release-branch.go1.15-security] cmd/go: overwrite program name with full path

If the program path is resolved, replace the first argument of the
exec.Cmd, which is the bare program name with the resolved path.

Change-Id: I92cf5e6f4bb7c8fef9b59f5eab963f4e75b90d07
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/957908
Reviewed-by: Katie Hockman <[email protected]>
Reviewed-by: Russ Cox <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>
(cherry picked from commit a863cb56b33a24aad88f23f1d48629dc4b4b9539)
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/958254
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: rolandshoemaker

src/cmd/go/internal/work/exec.go

@@ -1953,6 +1953,9 @@ func (b *Builder) runOut(a *Action, dir string, env []string, cmdargs ...interfa
 
 	var buf bytes.Buffer
 	cmd := exec.Command(cmdline[0], cmdline[1:]...)
+	if cmd.Path != "" {
+		cmd.Args[0] = cmd.Path
+	}
 	cmd.Stdout = &buf
 	cmd.Stderr = &buf
 	cleanup := passLongArgsInResponseFiles(cmd)