CI: pin to Ubuntu 20.04.

CI is currently broken because `ubuntu-latest` moved to 22.04, which is
missing at least one package (`libclang1-9` used in our CI jobs) and may
be causing other issues as well.

This PR pins us back to 20.04; separately we should look into upgrading
when issues are resolved.

.github/workflows/main.yml

@@ -19,7 +19,7 @@ jobs:
   # Check Code style quickly by running `rustfmt` over all code
   rustfmt:
     name: Rustfmt
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v3
       with:
@@ -32,7 +32,7 @@ jobs:
   # incompatible licences
   cargo_deny:
     name: Cargo deny
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v3
       with:
@@ -48,7 +48,7 @@ jobs:
   # Ensure dependencies are vetted. See https://mozilla.github.io/cargo-vet/
   cargo_vet:
     name: Cargo vet
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       CARGO_VET_VERSION: 0.3.0
     steps:
@@ -66,7 +66,7 @@ jobs:
 
   doc:
     name: Doc build
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       CARGO_MDBOOK_VERSION: 0.4.21
       RUSTDOCFLAGS: -Dbroken_intra_doc_links --cfg nightlydoc
@@ -135,7 +135,7 @@ jobs:
   # double-check that Rust code compiles and is likely to work everywhere else.
   checks:
     name: Check
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v3
       with:
@@ -189,7 +189,7 @@ jobs:
 
   fuzz_targets:
     name: Fuzz Targets
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v3
       with:
@@ -219,25 +219,25 @@ jobs:
     strategy:
       matrix:
         include:
-          - os: ubuntu-latest
+          - os: ubuntu-20.04
           # defaults to x86_64-apple-darwin
           - os: macos-latest
           - os: windows-latest
           - os: windows-latest
             target: x86_64-pc-windows-gnu
-          - os: ubuntu-latest
+          - os: ubuntu-20.04
             target: aarch64-unknown-linux-gnu
             gcc_package: gcc-aarch64-linux-gnu
             gcc: aarch64-linux-gnu-gcc
             qemu: qemu-aarch64 -L /usr/aarch64-linux-gnu
             qemu_target: aarch64-linux-user
-          - os: ubuntu-latest
+          - os: ubuntu-20.04
             target: s390x-unknown-linux-gnu
             gcc_package: gcc-s390x-linux-gnu
             gcc: s390x-linux-gnu-gcc
             qemu: qemu-s390x -L /usr/s390x-linux-gnu
             qemu_target: s390x-linux-user
-          - os: ubuntu-latest
+          - os: ubuntu-20.04
             target: riscv64gc-unknown-linux-gnu
             gcc_package: gcc-riscv64-linux-gnu
             gcc: riscv64-linux-gnu-gcc
@@ -265,7 +265,7 @@ jobs:
       with:
         path: ${{ runner.tool_cache }}/qemu
         key: qemu-${{ matrix.target }}-${{ env.QEMU_BUILD_VERSION }}-patchmadvise2
-      if: matrix.target != '' && matrix.os == 'ubuntu-latest'
+      if: matrix.target != '' && matrix.os == 'ubuntu-20.04'
     - name: Install cross-compilation tools
       run: |
         set -ex
@@ -328,14 +328,14 @@ jobs:
     - run: |
         sudo apt-get update && sudo apt-get install -y gdb lldb llvm
         cargo test test_debug_dwarf -- --ignored --test-threads 1
-      if: matrix.os == 'ubuntu-latest' && matrix.target == ''
+      if: matrix.os == 'ubuntu-20.04' && matrix.target == ''
       env:
         RUST_BACKTRACE: 1
 
   # Build and test the wasi-nn module.
   test_wasi_nn:
     name: Test wasi-nn module
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
         with:
@@ -350,7 +350,7 @@ jobs:
   # Build and test the wasi-crypto module.
   test_wasi_crypto:
     name: Test wasi-crypto module
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v3
       with:
@@ -363,7 +363,7 @@ jobs:
 
   bench:
     name: Run benchmarks
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v3
       with:
@@ -375,7 +375,7 @@ jobs:
   # Verify that cranelift's code generation is deterministic
   meta_determinist_check:
     name: Meta deterministic check
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v3
       with:
@@ -386,7 +386,7 @@ jobs:
 
   verify-publish:
     if: github.repository == 'bytecodealliance/wasmtime'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v3
       with: