ci: upgrade to gcc-11 on ubuntu-18.04

In #14150 , investigations showed
that using an older gcc to build the appimage could result in severe
perf issues. This is because older gcc versions happily replace calls to
libc functions with their own, which happen to be less optimized than
their libc counterparts.

We fix this problem by upgrading to gcc-11 on ubuntu-18.04.

.github/workflows/release.yml

@@ -13,6 +13,7 @@ on:
       - v[0-9]+.[0-9]+.[0-9]+
 
 # Build on the oldest supported images, so we have broader compatibility
+# Upgrade to gcc-11 to prevent it from using its builtins (#14150)
 jobs:
   linux:
     runs-on: ubuntu-18.04
@@ -26,11 +27,11 @@ jobs:
       - name: Install dependencies
         run: |
           sudo apt-get update
-          sudo apt-get install -y autoconf automake build-essential cmake gettext gperf libtool-bin locales ninja-build pkg-config unzip
+          sudo apt-get install -y autoconf automake build-essential cmake gcc-11 gettext gperf libtool-bin locales ninja-build pkg-config unzip
       - name: Build release
         id: build
         run: |
-          make CMAKE_BUILD_TYPE=RelWithDebInfo CMAKE_EXTRA_FLAGS="-DCMAKE_INSTALL_PREFIX:PATH="
+          CC=gcc-11 make CMAKE_BUILD_TYPE=RelWithDebInfo CMAKE_EXTRA_FLAGS="-DCMAKE_INSTALL_PREFIX:PATH="
           printf '::set-output name=version::%s\n' "$(./build/bin/nvim --version | head -n 3 | sed -z 's/\n/%0A/g')"
           printf '::set-output name=release::%s\n' "$(./build/bin/nvim --version | head -n 1)"
           make DESTDIR="$GITHUB_WORKSPACE/build/release/nvim-linux64" install
@@ -51,11 +52,11 @@ jobs:
       - name: Install dependencies
         run: |
           sudo apt-get update
-          sudo apt-get install -y autoconf automake build-essential cmake gettext gperf libtool-bin locales ninja-build pkg-config unzip
+          sudo apt-get install -y autoconf automake build-essential cmake gcc-11 gettext gperf libtool-bin locales ninja-build pkg-config unzip
       - if: github.event_name == 'push' || (github.event_name == 'workflow_dispatch' && github.event.inputs.tag_name != 'nightly')
-        run: make appimage-latest
+        run: CC=gcc-11 make appimage-latest
       - if: github.event_name == 'schedule' || (github.event_name == 'workflow_dispatch' && github.event.inputs.tag_name == 'nightly')
-        run: make appimage-nightly
+        run: CC=gcc-11 make appimage-nightly
       - uses: actions/upload-artifact@v2
         with:
           name: appimage