build: use RelWithDebInfo build for nightlies, Release for releases

Unlike Release build type, RelWithDebInfo does not disable asserts.
This helps get better debug info from people brave enough to use the
nightly builds, but shouldn't be used for official releases.

[skip ci]

.github/workflows/release.yml

@@ -28,10 +28,14 @@ jobs:
         run: |
           sudo apt-get update
           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: printf 'NVIM_BUILD_TYPE=Release\n' >> $GITHUB_ENV
+      - if: github.event_name == 'schedule' || (github.event_name == 'workflow_dispatch' && github.event.inputs.tag_name == 'nightly')
+        run: printf 'NVIM_BUILD_TYPE=RelWithDebInfo\n' >> $GITHUB_ENV
       - name: Build release
         id: build
         run: |
-          CC=gcc-11 make CMAKE_BUILD_TYPE=RelWithDebInfo CMAKE_EXTRA_FLAGS="-DCMAKE_INSTALL_PREFIX:PATH="
+          CC=gcc-11 make CMAKE_BUILD_TYPE=${NVIM_BUILD_TYPE} 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
@@ -80,9 +84,13 @@ jobs:
           brew update >/dev/null
           brew upgrade
           brew install automake ninja
+      - if: github.event_name == 'push' || (github.event_name == 'workflow_dispatch' && github.event.inputs.tag_name != 'nightly')
+        run: printf 'NVIM_BUILD_TYPE=Release\n' >> $GITHUB_ENV
+      - if: github.event_name == 'schedule' || (github.event_name == 'workflow_dispatch' && github.event.inputs.tag_name == 'nightly')
+        run: printf 'NVIM_BUILD_TYPE=RelWithDebInfo\n' >> $GITHUB_ENV
       - name: Build release
         run: |
-          make CMAKE_BUILD_TYPE=Release CMAKE_EXTRA_FLAGS="-DCMAKE_INSTALL_PREFIX:PATH= -DCMAKE_OSX_DEPLOYMENT_TARGET=10.11"
+          make CMAKE_BUILD_TYPE=${NVIM_BUILD_TYPE} CMAKE_EXTRA_FLAGS="-DCMAKE_INSTALL_PREFIX:PATH= -DCMAKE_OSX_DEPLOYMENT_TARGET=10.11"
           make DESTDIR="$GITHUB_WORKSPACE/build/release/nvim-osx64" install
       - name: Create package
         run: |