Update CI to Mesa 26.0.3.

Update `.github/actions/install-mesa/action.yml` to refer to
`build28`, which is Mesa 26.0.3. Update the generated ICD file's API
version, and update `file_format_version`.

Author: jimblandy

.github/actions/install-mesa/action.yml

@@ -4,10 +4,10 @@ inputs:
   # Sourced from https://archive.mesa3d.org/. Bumping this requires
   # updating the mesa build in https://github.com/gfx-rs/ci-build and creating a new release.
   version:
-    default: "25.2.7"
+    default: "26.0.3"
   # Corresponds to https://github.com/gfx-rs/ci-build/releases
   ci-binary-build:
-    default: "build26"
+    default: "build28"
   target-dir:
     description: "The directory into which to install the Mesa libraries."
     default: "target/llvm-cov-target/debug"
@@ -33,10 +33,10 @@ runs:
         cat <<- EOF > icd.json
         {
           "ICD": {
-              "api_version": "1.1.255",
+              "api_version": "1.4.335",
               "library_path": "$PWD/mesa/lib/x86_64-linux-gnu/libvulkan_lvp.so"
           },
-          "file_format_version": "1.0.0"
+          "file_format_version": "1.0.1"
         }
         EOF