ci: build multiprocess, fuzz and tsan depends with Clang

Even though TSan and fuzz don't need `MULTIPROCESS`, on GitLab we can
reuse depends while the current GitHub action *can't*, to maintain
similar behaviour between the two, the effective behaviour of the GitLab
configuration has been mirrored.

.github/workflows/build.yml

@@ -69,16 +69,16 @@ jobs:
           dep_opts: ""
         - build_target: linux64_multiprocess
           host: x86_64-pc-linux-gnu
-          dep_opts: "DEBUG=1 MULTIPROCESS=1"
+          dep_opts: "DEBUG=1 MULTIPROCESS=1 CC=clang-16 CXX=clang++-16"
         - build_target: linux64_tsan
           host: x86_64-pc-linux-gnu
-          dep_opts: "DEBUG=1"
+          dep_opts: "DEBUG=1 MULTIPROCESS=1 CC=clang-16 CXX=clang++-16"
         - build_target: linux64_ubsan
           host: x86_64-pc-linux-gnu
           dep_opts: "DEBUG=1"
         - build_target: linux64_fuzz
           host: x86_64-pc-linux-gnu
-          dep_opts: "DEBUG=1"
+          dep_opts: "DEBUG=1 MULTIPROCESS=1 CC=clang-16 CXX=clang++-16"
         - build_target: linux64_cxx20
           host: x86_64-pc-linux-gnu
           dep_opts: "DEBUG=1"

.gitlab-ci.yml

@@ -209,7 +209,7 @@ x86_64-pc-linux-gnu-multiprocess:
     - .skip-in-fast-mode-template
   variables:
     HOST: x86_64-pc-linux-gnu
-    DEP_OPTS: "DEBUG=1 MULTIPROCESS=1"
+    DEP_OPTS: "DEBUG=1 MULTIPROCESS=1 CC=clang-16 CXX=clang++-16"
 
 x86_64-apple-darwin:
   extends:
@@ -266,7 +266,7 @@ linux64_fuzz-build:
     - .build-template
     - .skip-in-fast-mode-template
   needs:
-    - x86_64-pc-linux-gnu-debug
+    - x86_64-pc-linux-gnu-multiprocess
   variables:
     BUILD_TARGET: linux64_fuzz
 
@@ -284,7 +284,7 @@ linux64_tsan-build:
     - .build-template
     - .skip-in-fast-mode-template
   needs:
-    - x86_64-pc-linux-gnu-debug
+    - x86_64-pc-linux-gnu-multiprocess
   variables:
     BUILD_TARGET: linux64_tsan