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
#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.

Also pinned the toolchain in the template app.

This change should be reverted once #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"
 
 # Install the `rust-src` component so that `-Zbuild-std` works. This in
 # addition to the components included in the default profile.

template/rust-toolchain.toml

@@ -1,3 +1,4 @@
 [toolchain]
-channel = "nightly"
+# Pinned due to https://github.com/rust-osdev/uefi-rs/issues/500.
+channel = "nightly-2022-09-01"
 components = ["rust-src"]