gh actions: don't fail fast (#1183)

by default, if any run of a matrix strategy fails, the others are
stopped immediately. In our case it means that if gcc fails, the
clang build will also fail, which is not what we want.

Change fail-fast to false so that if gcc fails, we can still see if
clang also fails or if it is a gcc specific problem or vice versa.

.github/workflows/build-pr.yml

@@ -83,6 +83,7 @@ jobs:
     runs-on: ubuntu-latest
     name: linux (${{ matrix.cc }})
     strategy:
+      fail-fast: false
       matrix:
         include:
         - cc: gcc

.github/workflows/build.yml

@@ -84,6 +84,7 @@ jobs:
     runs-on: ubuntu-latest
     name: linux (${{ matrix.cc }})
     strategy:
+      fail-fast: false
       matrix:
         include:
         - cc: gcc