Use force checkout

Even though I don't know of a situation where this would be needed, it is how it was done before the switch to using
`github.com/go-git/go-git`:

https://github.com/bcmi-labs/arduino-modules/blob/v1.1.12/git/git_helper.go#L93

Since the previous system was working well, it seems best to emulate its behavior as closely as possible.

Author: per1234

libraries/git_integration_test.go

@@ -45,7 +45,7 @@ func TestUpdateLibraryJson(t *testing.T) {
 		// Annotated tags have their own hash, different from the commit hash, so the tag must be resolved before checkout
 		resolvedTag, err := r.Repository.ResolveRevision(plumbing.Revision(tag.Hash().String()))
 		require.NoError(t, err)
-		err = repoTree.Checkout(&git.CheckoutOptions{Hash: *resolvedTag})
+		err = repoTree.Checkout(&git.CheckoutOptions{Hash: *resolvedTag, Force: true})
 		require.NoError(t, err)
 
 		library, err := GenerateLibraryFromRepo(r)

sync_libraries.go

@@ -240,7 +240,7 @@ func syncLibraryTaggedRelease(logger *log.Logger, repo *libraries.Repository, ta
 		panic(err)
 	}
 
-	if err = repoTree.Checkout(&git.CheckoutOptions{Hash: *resolvedTag}); err != nil {
+	if err = repoTree.Checkout(&git.CheckoutOptions{Hash: *resolvedTag, Force: true}); err != nil {
 		return fmt.Errorf("Error checking out repo: %s", err)
 	}