cmd/go: replace some calls to base.AppendPWD with cmd.Environ

With #50599 implemented, base.AppendPWD is redundant if cmd.Env would
otherwise be nil, and calls to os.Environ followed by base.AppendPWD
can be replaced by a simpler call to cmd.Environ.

Updates #50599.

Change-Id: I94a22e2a4cc8e83c815ac41702ea0b1ee5034ecc
Reviewed-on: https://go-review.googlesource.com/c/go/+/401534
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/cmd/go/internal/vcs/vcs.go

@@ -22,7 +22,6 @@ import (
 	"sync"
 	"time"
 
-	"cmd/go/internal/base"
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/search"
 	"cmd/go/internal/str"
@@ -657,7 +656,6 @@ func (v *Cmd) run1(dir string, cmdline string, keyval []string, verbose bool) ([
 
 	cmd := exec.Command(v.Cmd, args...)
 	cmd.Dir = dir
-	cmd.Env = base.AppendPWD(os.Environ(), cmd.Dir)
 	if cfg.BuildX {
 		fmt.Fprintf(os.Stderr, "cd %s\n", dir)
 		fmt.Fprintf(os.Stderr, "%s %s\n", v.Cmd, strings.Join(args, " "))

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

@@ -2116,11 +2116,10 @@ func (b *Builder) runOut(a *Action, dir string, env []string, cmdargs ...any) ([
 	cmd.Stderr = &buf
 	cleanup := passLongArgsInResponseFiles(cmd)
 	defer cleanup()
-	cmd.Env = os.Environ()
 	if dir != "." {
 		cmd.Dir = dir
-		cmd.Env = base.AppendPWD(cmd.Env, dir)
 	}
+	cmd.Env = cmd.Environ() // Pre-allocate with correct PWD.
 
 	// Add the TOOLEXEC_IMPORTPATH environment variable for -toolexec tools.
 	// It doesn't really matter if -toolexec isn't being used.
@@ -2609,8 +2608,7 @@ func (b *Builder) gccSupportsFlag(compiler []string, flag string) bool {
 	}
 	cmd := exec.Command(cmdArgs[0], cmdArgs[1:]...)
 	cmd.Dir = b.WorkDir
-	cmd.Env = base.AppendPWD(os.Environ(), cmd.Dir)
-	cmd.Env = append(cmd.Env, "LC_ALL=C")
+	cmd.Env = append(cmd.Environ(), "LC_ALL=C")
 	out, _ := cmd.CombinedOutput()
 	// GCC says "unrecognized command line option".
 	// clang says "unknown argument".