ci: Temporarily pin to older nightly

There are a bunch of new lint errors that need to be addressed before we can
successfully build with latest nightly.

Author: nicholasbishop

.github/workflows/nightly_toolchain.toml

@@ -1,3 +1,3 @@
 [toolchain]
-channel = "nightly"
+channel = "nightly-2025-06-01"
 targets = ["aarch64-unknown-uefi", "i686-unknown-uefi", "x86_64-unknown-uefi"]