github: Shorten the names of some jobs

This makes it easier to view the details of each individual job
in the matrix of each of these, in the github UI.

Signed-off-by: Martin Storsjö <martin@martin.st>

.github/workflows/build.yml

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 778e42e..59d8ded 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -232,7 +232,7 @@
           ninja
           ninja check-runtimes
 
-  linux-test-cross-build-ffmpeg:
+  test-cross-build-ffmpeg:
     needs: [llvm-mingw]
     runs-on: ubuntu-latest
     strategy:
@@ -552,7 +552,7 @@
           g++ test/hello-exception.cpp -o hello-exception.exe
           ./hello-exception.exe
 
-  mingw-w64-testcases-clang:
+  testcases-clang:
     defaults:
       run:
         shell: msys2 {0}
@@ -604,7 +604,7 @@
           make -j$(nproc) install
           make -j$(nproc) check -k -O || { cat testcases/test-suite.log; exit 1; }
 
-  mingw-w64-testcases-gcc:
+  testcases-gcc:
     runs-on: windows-latest
     needs: [gcc-cross]
     defaults: