Bump fuzz MSRV to 1.80

honggfuzz builds appear to fail on 1.75 - 1.79, so we just pick an
MSRV of the first toolchain that actually works.

Sadly, github for some reason drops a trailing in the env entry, so
we copy the rust version a few places.

Author: TheBlueMatt

.github/workflows/build.yml

@@ -253,15 +253,15 @@ jobs:
 
   fuzz:
     runs-on: self-hosted
-    env:
-      TOOLCHAIN: 1.75
     steps:
       - name: Checkout source code
         uses: actions/checkout@v4
-      - name: Install Rust ${{ env.TOOLCHAIN }} toolchain
+      # For whatever reason, honggfuzz doesn't build on 1.75, and there's not a lot of
+      # reason to insist on 1.75 for fuzzing, so we just pick an MSRV of 1.80 for fuzz.
+      - name: Install Rust 1.80 toolchain
         run: |
-          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile=minimal --default-toolchain ${{ env.TOOLCHAIN }}
-      - name: Sanity check fuzz targets on Rust ${{ env.TOOLCHAIN }}
+          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile=minimal --default-toolchain 1.80
+      - name: Sanity check fuzz targets on Rust 1.80
         run: |
           cd fuzz
           RUSTFLAGS="--cfg=fuzzing --cfg=secp256k1_fuzz --cfg=hashes_fuzz" cargo test --verbose --color always --lib --bins -j8