ci+fixtures: UCRT64 msys2 env; fix remaining fixture paths

ci (Windows): the runner's msys2 shell starts as the base MSYS
environment (MSYSTEM=MSYS), which install-deps.sh rejects. Set the
job-level MSYSTEM=UCRT64 env and prepend /ucrt64/bin to PATH in every
Windows step, so pacman installs and the toolchain resolve against the
mingw-w64-ucrt-x86_64 packages.

fixtures: the restructured workspace moved the media fixtures into
crates/oak-app/tests; the remaining references pointed at the old
repo-root tests/ — oak-codec realmedia_tests + hwdecode, oak-node
serializer golden, oak-worker procpool integration. All point at
../oak-app/tests now (oak-app's own tests/demo.mp4 references were
already correct after the move).

crates/oak-node/tests/serializer_test.rs

@@ -155,7 +155,7 @@ fn golden_project_with_footage_loads() {
 
 	let xml = std::fs::read_to_string(concat!(
 		env!("CARGO_MANIFEST_DIR"),
-		"/../../tests/project_with_footage.ove"
+		"/../oak-app/tests/project_with_footage.ove"
 	))
 	.unwrap();
 	let project = oak_node::serializer::load(&xml).unwrap();