Fill the module cache in go.work vex fixtures

vex reads which version a go.work workspace selects from its module
graph in the module cache, and since its go.work replaces are checked
from the workspace root, the go.work flavor of the lockfile fixtures
left them out with go_version_not_selected. The fixture now puts the
patched module's go.mod in its private cache, as `go mod download`
would.

Assisted-by: Claude Code:claude-opus-5-5

Author: mikolalysenko

crates/socket-patch-cli/tests/e2e_vex_lockfile/golang.rs

@@ -497,6 +497,7 @@ fn write_go_replace(fx: &Fx, file: GoFile, rhs: &str, sum: Option<&str>) {
         }
         GoFile::GoWork => {
             fx.put("go.mod", go_mod(GO_VERSION, None, false));
+            fx.cache_go_mod(GO_MODULE, GO_VERSION);
             fx.put(
                 "go.work",
                 format!("go 1.21\n\nuse .\n\nreplace {GO_MODULE} {GO_VERSION} => {rhs}\n"),