Revert "Use ubuntu-latest"

This reverts commit 5ac342c.

It seems they haven't switched to 24.04 yet:
actions/runner-images#10636
Leave it as-is for now and we can revisit this if needed.

Signed-off-by: Nick Kossifidis <mick@ics.forth.gr>

.github/workflows/build.yaml

@@ -53,7 +53,7 @@ jobs:
     needs: [submodule_cache]
     strategy:
       matrix:
-        os:     [ubuntu-latest]
+        os:     [ubuntu-22.04, ubuntu-24.04]
         mode:   [newlib, linux, musl, uclibc]
         target: [rv32gc-ilp32d, rv64gc-lp64d]
         compiler: [gcc, llvm]
@@ -98,7 +98,8 @@ jobs:
 
       - name: make report
         if: |
-          (matrix.mode == 'linux' || matrix.mode == 'newlib')
+          matrix.os == 'ubuntu-24.04'
+          && (matrix.mode == 'linux' || matrix.mode == 'newlib')
           && matrix.compiler == 'gcc'
         run: |
           make report-${{ matrix.mode }} -j $(nproc)
@@ -135,7 +136,7 @@ jobs:
     needs: [submodule_cache]
     strategy:
       matrix:
-        os:     [ubuntu-latest]
+        os:     [ubuntu-24.04]
         mode:   [newlib]
         target: [rv64gc-lp64d]
         sim:    [spike]
@@ -171,7 +172,7 @@ jobs:
     needs: [submodule_cache]
     strategy:
       matrix:
-        os:     [ubuntu-latest]
+        os:     [ubuntu-24.04]
         mode:   [newlib, linux]
         target: [rv64gc-lp64d]
     steps: