fix(agent): SHA256SUMS being looked up in the parent directory (#842)

We didn't notice this behaviour in tests because our mocked file server
was serving files in the root directory (since the parent of / is /
everything just worked).

Changing tests to serve and search files inside a subdirectory reveal
the problem: we're not looking for the checksums file beside the rootfs
but rather in the parent directory. We see log lines like below:

> time="2024-07-18T10:42:53-03:00" level=debug msg="Landscape: received
command Install. Target:
testDistro_UP4W_TestInstall_Error_when_the_checksum_entry_is_missing_for_the_rootfs_5721719220902747386"
> time="2024-07-18T10:42:53-03:00" level=info msg="checksums file
http://127.0.0.1:57850/releases/SHA256SUM" not found"

---
UDENG-3669

windows-agent/internal/proservices/landscape/executor.go

@@ -334,7 +334,7 @@ func download(ctx context.Context, u *url.URL, destination string) (err error) {
 // ...
 func wantRootfsChecksum(ctx context.Context, u *url.URL) (string, error) {
 	imageName := filepath.Base(u.Path)
-	shasRelativeURL, err := url.Parse("../SHA256SUMS")
+	shasRelativeURL, err := url.Parse("SHA256SUMS")
 	if err != nil {
 		return "", fmt.Errorf("could not assemble SHA256SUMS location: %v", err)
 	}

windows-agent/internal/proservices/landscape/executor_test.go

@@ -279,7 +279,7 @@ func TestInstall(t *testing.T) {
 					u := tc.sendRootfsURL
 					var err error
 					if tc.sendRootfsURL != brokenURL {
-						u, err = url.JoinPath(fileServerAddr, tc.sendRootfsURL)
+						u, err = url.JoinPath(fileServerAddr, "/releases/theone", tc.sendRootfsURL)
 						require.NoError(t, err, "Setup: could not assemble URL: %s + %s", fileServerAddr, tc.sendRootfsURL)
 					}
 
@@ -329,20 +329,22 @@ func mockRootfsFileServer(t *testing.T, ctx context.Context, enableChecksumsFile
 
 	mux := http.NewServeMux()
 
-	mux.HandleFunc("GET /goodfile", func(w http.ResponseWriter, r *http.Request) {})             // Return empty file
-	mux.HandleFunc("GET /badchecksum", func(w http.ResponseWriter, r *http.Request) {})          // Return empty file
-	mux.HandleFunc("GET /rootfswithnochecksum", func(w http.ResponseWriter, r *http.Request) {}) // intentionally not in the checksums file
-	mux.HandleFunc("GET /badfile", func(w http.ResponseWriter, r *http.Request) {
+	const getFile = "GET /releases/theone/%s"
+
+	mux.HandleFunc(fmt.Sprintf(getFile, "goodfile"), func(w http.ResponseWriter, r *http.Request) {})             // Return empty file
+	mux.HandleFunc(fmt.Sprintf(getFile, "badchecksum"), func(w http.ResponseWriter, r *http.Request) {})          // Return empty file
+	mux.HandleFunc(fmt.Sprintf(getFile, "rootfswithnochecksum"), func(w http.ResponseWriter, r *http.Request) {}) // intentionally not in the checksums file
+	mux.HandleFunc(fmt.Sprintf(getFile, "badfile"), func(w http.ResponseWriter, r *http.Request) {
 		_, err := fmt.Fprintf(w, "MOCK_ERROR")
 		if err != nil {
 			t.Logf("mockRootfsFileServer: could not write response: %v", err)
 		}
 	})
-	mux.HandleFunc("GET /badresponse", func(w http.ResponseWriter, r *http.Request) {
+	mux.HandleFunc(fmt.Sprintf(getFile, "badresponse"), func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusNotFound)
 	})
 	if enableChecksumsFile {
-		mux.HandleFunc("GET /SHA256SUMS", func(w http.ResponseWriter, r *http.Request) {
+		mux.HandleFunc(fmt.Sprintf(getFile, "SHA256SUMS"), func(w http.ResponseWriter, r *http.Request) {
 			_, err := fmt.Fprintf(w, `e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 *goodfile
 		afe55cda4210c2439b47c62c01039027522f7ed4abdb113972b3030b3359532a *badfile
 		1234 *badchecksum