[release-branch.go1.21] cmd/go/internal/generate: call modload.InitWorkFile

This is necessary for go generate to enter workspace mode for
recognizing package paths in the workspace.

For #56098
Fixes #65351

Change-Id: I25f68de24f4189259353f63194823516e9d3d505
Reviewed-on: https://go-review.googlesource.com/c/go/+/559195
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
(cherry picked from commit b91bad7819)
Reviewed-on: https://go-review.googlesource.com/c/go/+/559235

src/cmd/go/internal/generate/generate.go

@@ -181,6 +181,8 @@ func init() {
 }
 
 func runGenerate(ctx context.Context, cmd *base.Command, args []string) {
+	modload.InitWorkfile()
+
 	if generateRunFlag != "" {
 		var err error
 		generateRunRE, err = regexp.Compile(generateRunFlag)

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

@@ -0,0 +1,27 @@
+# This is a regression test for Issue #56098: Go generate
+# wasn't initializing workspace mode
+
+[short] skip
+
+go generate ./mod
+cmp ./mod/got.txt want.txt
+
+-- go.work --
+go 1.21
+
+use ./mod
+-- mod/go.mod --
+module example.com/mod
+-- mod/gen.go --
+//go:generate go run gen.go got.txt
+
+package main
+
+import "os"
+
+func main() {
+    outfile := os.Args[1]
+    os.WriteFile(outfile, []byte("Hello World!\n"), 0644)
+}
+-- want.txt --
+Hello World!