ci: run on older ubuntu

From guythatsits:

> Not sure if this is helpful but the reason it isn't installing in any
of the debian based is the libc6 dependency(requires 2.39, these are
both on 2.35). Looks like popos alpha apparently includes the updated
library. Had the same issue on just a barebone debian system as well.

By default, GitHub Actions uses the latest Ubuntu runner (e.g.,
ubuntu-latest), which has glibc 2.39+. Instead, we can explicitly use
ubuntu-20.04, which has glibc 2.31.

Reported-by: guythatsits
Fixes: https://github.com/damus-io/notedeck/issues/706
Changelog-Fixed: Fixed issue running binary on older debian distros
Signed-off-by: William Casarin <jb55@jb55.com>

.github/workflows/rust.yml

@@ -12,7 +12,7 @@ on:
 jobs:
   lint: 
     name: Rustfmt + Clippy
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable
@@ -26,7 +26,7 @@ jobs:
     name: Test (Linux)
     uses: ./.github/workflows/build-and-test.yml
     with:
-      os: ubuntu-latest
+      os: ubuntu-20.04
       additional-setup: |
         sudo apt-get install libxcb-render0-dev libxcb-shape0-dev libxcb-xfixes0-dev libspeechd-dev libxkbcommon-dev libssl-dev
 
@@ -44,7 +44,7 @@ jobs:
 
   packaging:
     name: rpm/deb
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     needs: linux-test
     if: github.ref_name == 'master' || github.ref_name == 'ci'
 
@@ -71,6 +71,9 @@ jobs:
       - name: Rust cache
         uses: Swatinem/rust-cache@v2
 
+      - name: Cargo Fetch
+        run: cargo fetch
+
       - name: Build Cross (${{ matrix.arch }})
         if: matrix.arch != runner.arch
         run: cargo build --release --target=${{ matrix.arch }}-unknown-linux-gnu