ci: add comment about backends which do not have nopanic checks (#530)

Also switch rust toolchain from Nightly to Stable in the nopanic's linux job.

.github/workflows/nopanic.yaml

@@ -1,3 +1,12 @@
+# This CI config checks that  getrandom's backend implementation
+# do not contain potential panics.
+#
+# It is unclear how to implement a no-panic check for esp_idf, fuchsia,
+# vxworks, hermit, and solid backends, so we do not check them here,
+# but they still should be panic-free. We do not check wasm_js backend
+# since glue code generated by wasm-bindgen contains potential panics,
+# so resulting WASM files inevitably contain potential panics for any
+# code which non-trivially interacts with the JS enviroment.
 name: No panic
 
 on:
@@ -24,8 +33,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master
         with:
-          # We need Nightly for the rust-std component for wasm32-wasip2
-          toolchain: nightly-2024-10-14
+          toolchain: stable
           targets: wasm32-wasip1, wasm32-wasip2
       - uses: Swatinem/rust-cache@v2