ci: inline internal caching script to Github actions (#22192)

I don't think it's possible to meaningfully abstract away caching on
multiple providers, as each provider has different mechanisms
on how they work.

.github/workflows/ci.yml

@@ -110,9 +110,6 @@ jobs:
         run: |
           git diff --color --exit-code
 
-      - name: Cache dependencies
-        run: ./ci/before_cache.sh
-
   lintc:
     # This job tests two things: it lints the code but also builds neovim using
     # system dependencies instead of bundled dependencies. This is to make sure
@@ -173,9 +170,6 @@ jobs:
       - if: success() || failure() && steps.abort_job.outputs.status == 'success'
         run: cmake --build build --target clang-tidy
 
-      - name: Cache dependencies
-        run: ./ci/before_cache.sh
-
   posix:
     name: ${{ matrix.runner }} ${{ matrix.flavor }} (cc=${{ matrix.cc }})
     strategy:
@@ -282,9 +276,6 @@ jobs:
         name: Install nvim
         run: ./ci/run_tests.sh install_nvim
 
-      - name: Cache dependencies
-        run: ./ci/before_cache.sh
-
   windows:
     runs-on: windows-2019
     timeout-minutes: 45

.github/workflows/env.sh

@@ -11,7 +11,7 @@ LOG_DIR=$BUILD_DIR/log
 NVIM_LOG_FILE=$BUILD_DIR/.nvimlog
 VALGRIND_LOG=$LOG_DIR/valgrind-%p.log
 CACHE_DIR=$HOME/.cache
-CACHE_NVIM_DEPS_DIR=$CACHE_DIR/nvim-deps
+CACHE_NVIM_DEPS_DIR=$DEPS_BUILD_DIR
 CACHE_MARKER=$CACHE_NVIM_DEPS_DIR/.ci_cache_marker
 CACHE_UNCRUSTIFY=$CACHE_DIR/uncrustify
 DEPS_CMAKE_FLAGS=