cmd/go: include AR env var in gccgo build IDs

The gccgo toolchain uses the archiver specified by the AR environment
variable, or `ar` by default. Teach the build ID to take the value of
this environment variable into account, since different archivers can
produce different results.

Fix #30046.

Change-Id: Ia6821258d54eecedb9026afc38a515cd564c45cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/160897
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -8,6 +8,11 @@ package work
 
 import (
 	"bytes"
+	"cmd/go/internal/base"
+	"cmd/go/internal/cache"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+	"cmd/go/internal/str"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -25,12 +30,6 @@ import (
 	"strings"
 	"sync"
 	"time"
-
-	"cmd/go/internal/base"
-	"cmd/go/internal/cache"
-	"cmd/go/internal/cfg"
-	"cmd/go/internal/load"
-	"cmd/go/internal/str"
 )
 
 // actionList returns the list of actions in the dag rooted at root
@@ -270,6 +269,7 @@ func (b *Builder) buildActionID(a *Action) cache.ActionID {
 		}
 		fmt.Fprintf(h, "compile %s %q %q\n", id, forcedGccgoflags, p.Internal.Gccgoflags)
 		fmt.Fprintf(h, "pkgpath %s\n", gccgoPkgpath(p))
+		fmt.Fprintf(h, "ar %q\n", BuildToolchain.(gccgoToolchain).ar())
 		if len(p.SFiles) > 0 {
 			id, _ = b.gccgoToolID(BuildToolchain.compiler(), "assembler-with-cpp")
 			// Ignore error; different assembler versions