build(rust): bump to Rust 1.91.0 (#10767)

Rust 1.91 has been released and brings with it a few new lints that we
need to tidy up. In addition, it also stabilizes `BTreeMap::extract_if`:
A really nifty std-lib function that allows us to conditionally take
elements from a map. We need that in a bunch of places.

.github/actions/setup-rust/action.yml

@@ -74,10 +74,10 @@ runs:
         echo "SCCACHE_AZURE_BLOB_CONTAINER=sccache" >> $GITHUB_ENV
         echo "RUSTC_WRAPPER=$SCCACHE_PATH" >> $GITHUB_ENV
 
-    - name: Install nightly Rust
+    - name: Install nightly Rust to use for certain tools (fuzzing, cargo-udeps)
       id: nightly
       run: |
-        NIGHTLY="nightly-2025-05-30"
+        NIGHTLY="nightly-2025-09-30"
         # Check if nightly toolchain is already installed
         if ! rustup toolchain list | grep -q "$NIGHTLY"; then
           rustup toolchain install $NIGHTLY
@@ -86,6 +86,16 @@ runs:
         echo "nightly=$NIGHTLY" >> $GITHUB_OUTPUT
       shell: bash
 
+    - name: Install nightly Rust for compiling eBPF code
+      run: |
+        NIGHTLY="nightly-2025-05-30"
+        # Check if nightly toolchain is already installed
+        if ! rustup toolchain list | grep -q "$NIGHTLY"; then
+          rustup toolchain install $NIGHTLY
+          rustup component add rust-src --toolchain $NIGHTLY
+        fi
+      shell: bash
+
     - name: Start sccache
       run: $SCCACHE_PATH --start-server
       shell: bash