ci: don't automatically enable -Werror if in CI environment

This catches downstream consumers of neovim off-guard when trying to use
neovim in an esoteric environment not tested in our own CI.

Closes https://github.com/neovim/neovim/issues/22932

(cherry picked from commit 0256b67e89)

.github/workflows/build.yml

@@ -67,7 +67,7 @@ jobs:
         run: make deps
 
       - name: Build
-        run: make CMAKE_FLAGS="-D CMAKE_INSTALL_PREFIX:PATH=$INSTALL_PREFIX"
+        run: make CMAKE_FLAGS="-D CI_BUILD=ON -D CMAKE_INSTALL_PREFIX:PATH=$INSTALL_PREFIX"
 
       - name: Install
         run: make install

.github/workflows/release.yml

@@ -33,7 +33,7 @@ jobs:
       - name: Build release
         id: build
         run: |
-          CC=gcc-10 make CMAKE_BUILD_TYPE=${NVIM_BUILD_TYPE} CMAKE_EXTRA_FLAGS="-DCI_BUILD=OFF -DCMAKE_INSTALL_PREFIX:PATH="
+          CC=gcc-10 make CMAKE_BUILD_TYPE=${NVIM_BUILD_TYPE} CMAKE_EXTRA_FLAGS="-DCMAKE_INSTALL_PREFIX:PATH="
           printf 'version<<END\n' >> $GITHUB_OUTPUT
           ./build/bin/nvim --version | head -n 3 >> $GITHUB_OUTPUT
           printf 'END\n' >> $GITHUB_OUTPUT
@@ -126,7 +126,7 @@ jobs:
           cmake --build .deps
       - name: build package
         run: |
-          cmake -B build -G Ninja -DCMAKE_BUILD_TYPE='RelWithDebInfo' -D CI_BUILD=OFF
+          cmake -B build -G Ninja -DCMAKE_BUILD_TYPE='RelWithDebInfo'
           cmake --build build --target package
       - uses: actions/upload-artifact@v3
         with:

.github/workflows/test.yml

@@ -76,7 +76,7 @@ jobs:
 
       - if: success() || failure() && steps.abort_job.outputs.status == 'success'
         name: configure
-        run: cmake -B build -G Ninja -D CI_BUILD=OFF
+        run: cmake -B build -G Ninja
 
       - if: "!cancelled()"
         name: Determine if run should be aborted
@@ -197,7 +197,7 @@ jobs:
 
       - name: Build
         run: |
-          cmake -B build -G Ninja -D CMAKE_INSTALL_PREFIX:PATH=$INSTALL_PREFIX ${{ matrix.flags }}
+          cmake -B build -G Ninja -D CMAKE_INSTALL_PREFIX:PATH=$INSTALL_PREFIX ${{ matrix.flags }} -D CI_BUILD=ON
           cmake --build build
 
       - if: "!cancelled()"
@@ -276,7 +276,7 @@ jobs:
           cmake --build .deps
 
       - name: Configure
-        run: cmake -B build -G "Ninja Multi-Config" -D CMAKE_C_COMPILER=gcc
+        run: cmake -B build -G "Ninja Multi-Config" -D CMAKE_C_COMPILER=gcc -D CI_BUILD=ON
 
       - name: Release
         run: cmake --build build --config Release
@@ -313,7 +313,7 @@ jobs:
 
       - name: Build
         run: |
-          cmake -B build -G Ninja -D CMAKE_BUILD_TYPE='RelWithDebInfo'
+          cmake -B build -G Ninja -D CMAKE_BUILD_TYPE='RelWithDebInfo' -D CI_BUILD=ON
           cmake --build build
 
       - name: Install test deps
@@ -405,5 +405,5 @@ jobs:
 
       - name: Build
         run: |
-          cmake -B build -G Ninja
+          cmake -B build -G Ninja -D CI_BUILD=ON
           cmake --build build