cmd/dist: simplify exec.Cmd helpers

Now that the required Go bootstrap version is 1.17, we can simplify
some of the logic for working with exec.Cmd.Env as those APIs have
been simplified.

Updates #44505.

Change-Id: Ia62ab9830320c7daf65a15f2313471dfaba687b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/427957
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/cmd/dist/exec.go

@@ -17,23 +17,11 @@ func setDir(cmd *exec.Cmd, dir string) {
 }
 
 // setEnv sets cmd.Env so that key = value.
-//
-// It first removes any existing values for key, so it is safe to call
-// even from within cmdbootstrap.
 func setEnv(cmd *exec.Cmd, key, value string) {
 	kv := key + "=" + value
 	if cmd.Env == nil {
 		cmd.Env = os.Environ()
 	}
-
-	prefix := kv[:len(key)+1]
-	for i, entry := range cmd.Env {
-		if strings.HasPrefix(entry, prefix) {
-			cmd.Env[i] = kv
-			return
-		}
-	}
-
 	cmd.Env = append(cmd.Env, kv)
 }
 
@@ -44,10 +32,13 @@ func unsetEnv(cmd *exec.Cmd, key string) {
 	}
 
 	prefix := key + "="
-	for i, entry := range cmd.Env {
+	newEnv := []string{}
+	for _, entry := range cmd.Env {
 		if strings.HasPrefix(entry, prefix) {
-			cmd.Env = append(cmd.Env[:i], cmd.Env[i+1:]...)
-			return
+			continue
 		}
+		newEnv = append(newEnv, entry)
+		// key may appear multiple times, so keep going.
 	}
+	cmd.Env = newEnv
 }