cmd/go: run go install in workspace mode

It's too confusing to users to run go install in module mode, so run
it in workspace mode instead.

Fixes #50036

Change-Id: Ia99927bd98f54be4c42224a247543892045e3464
Reviewed-on: https://go-review.googlesource.com/c/go/+/377334
Reviewed-by: Bryan Mills <bcmills@google.com>
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/work/build.go

@@ -617,6 +617,7 @@ func runInstall(ctx context.Context, cmd *base.Command, args []string) {
 		}
 	}
 
+	modload.InitWorkfile()
 	BuildInit()
 	pkgs := load.PackagesAndErrors(ctx, load.PackageOpts{}, args)
 	if cfg.ModulesEnabled && !modload.HasModRoot() {

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

@@ -0,0 +1,36 @@
+# This is a regression test for golang.org/issue/50036
+# Don't check sums for other modules in the workspace.
+
+cd m/sub
+go install -n
+
+-- go.work --
+go 1.18
+
+use (
+    ./m
+    ./m/sub
+)
+-- m/go.mod --
+module example.com/m
+
+go 1.18
+
+-- m/m.go --
+package m
+
+func M() {}
+-- m/sub/go.mod --
+module example.com/m/sub
+
+go 1.18
+
+require example.com/m v1.0.0
+-- m/sub/main.go --
+package main
+
+import "example.com/m"
+
+func main() {
+    m.M()
+}