Pin to a working nightly

Temporarily modify rust-toolchain.toml to pin to `nightly-2022-08-26`,
as compilation is broken on current nightly due to
rust-osdev#500. `nightly-2022-09-01`
is the most recent nightly that avoids the bug, but it doesn't have miri
(which is run by CI), so pin to a slightly older version.

This change should be reverted once rust-osdev#500 is fixed.

rust-toolchain.toml

@@ -1,6 +1,9 @@
 [toolchain]
-# Nightly is required due to use of unstable features.
-channel = "nightly"
+# Pinned due to https://github.com/rust-osdev/uefi-rs/issues/500.
+#
+# Compilation started failing 2022-09-02. The previous several builds
+# don't have miri available though, so pin to a slightly older version.
+channel = "nightly-2022-08-26"
 components = [
     # Needed for `cargo xtask miri`.
     "miri",