Pin CI runner images to explicit versions

CI is run infrequently enough on this repository that there's a
nontrivial chance that GitHub updates the definition of `ubuntu-latest`
for example between two CI runs. This can be confusing for contributors
as PRs seemingly break CI when in reality they have nothing to do with
the breakage and it's due to the image changing.

This PR pins all images to the definition of `*-latest` at this time.
The one exception is that one builder was pinned to 22.04 and I'm going
to update it to 24.04 here and see if I can't fix CI issues that come up.

This'll require explicit PRs to update these images in the future, but
hopefully that's only once every few ~years so not too much of a burden.

.github/workflows/main.yml

@@ -19,10 +19,10 @@ jobs:
       matrix:
         include:
           - artifact: x86_64-linux
-            os: ubuntu-latest
+            os: ubuntu-24.04
 
           - artifact: arm64-linux
-            os: ubuntu-latest
+            os: ubuntu-24.04
             rust_target: aarch64-unknown-linux-gnu
             env:
               # Don't build a sysroot for this cross-compiled target since it
@@ -41,7 +41,7 @@ jobs:
                 -DRUST_TARGET=aarch64-unknown-linux-gnu
 
           - artifact: arm64-macos
-            os: macos-latest
+            os: macos-14
             rust_target: aarch64-apple-darwin
             env:
               WASI_SDK_CI_TOOLCHAIN_LLVM_CMAKE_ARGS: >-
@@ -49,7 +49,7 @@ jobs:
                 -DCMAKE_OSX_ARCHITECTURES=arm64
 
           - artifact: x86_64-macos
-            os: macos-latest
+            os: macos-14
             rust_target: x86_64-apple-darwin
             env:
               WASI_SDK_CI_SKIP_SYSROOT: 1
@@ -58,7 +58,7 @@ jobs:
                 -DCMAKE_OSX_ARCHITECTURES=x86_64
 
           - artifact: x86_64-windows
-            os: windows-latest
+            os: windows-2022
 
     env: ${{ matrix.env || fromJSON('{}') }}
     steps:
@@ -165,7 +165,7 @@ jobs:
 
   build-only-sysroot:
     name: Build only sysroot
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v4
         with:
@@ -193,7 +193,7 @@ jobs:
   finalize:
     name: Finalize wasi-sdk artifacts
     needs: build
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v4
         with:
@@ -253,7 +253,7 @@ jobs:
   test-standalone:
     name: Test standalone toolchain
     needs: build
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v4
         with: