cmd/go/internal/tool: set Internal.ExeName on tool's package

While the cached name of an executable is set based on the base name of
the package path, the executable produced as the output of link doesn't
have ExeName set on it and is just called a.out (with a .exe suffix on
Windows). Set ExeName so that the first time the binary is run, from the
directory link is run in, it has the right name for ps.

For #48429

Change-Id: Ic049304ec6fd5b23c2f5aaaf91aa58d79fe5a7ba
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/630695
Reviewed-by: Conrad Irwin <[email protected]>
Reviewed-by: Hongxiang Jiang <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

src/cmd/go/internal/tool/tool.go

@@ -287,6 +287,7 @@ func buildAndRunModtool(ctx context.Context, tool string, args []string) {
 	pkgOpts := load.PackageOpts{MainOnly: true}
 	p := load.PackagesAndErrors(ctx, pkgOpts, []string{tool})[0]
 	p.Internal.OmitDebug = true
+	p.Internal.ExeName = path.Base(p.ImportPath)
 
 	a1 := b.LinkAction(work.ModeBuild, work.ModeBuild, p)
 	a1.CacheExecutable = true

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

@@ -0,0 +1,32 @@
+[short] skip 'runs go build'
+
+# First run: executable for bar is not cached.
+# Make sure it's not called a.out
+go tool bar
+stdout 'my name is: bar'$GOEXE
+! stdout 'a.out'
+
+# Second run: executable is cached. Make sure it
+# has the right name.
+go tool bar
+stdout 'my name is: bar'$GOEXE
+! stdout 'a.out'
+
+-- go.mod --
+module example.com/foo
+
+go 1.24
+
+tool example.com/foo/bar
+-- bar/bar.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+func main() {
+	fmt.Println("my name is:", filepath.Base(os.Args[0]))
+}