ci: automatically maximize MIN_LOG_LEVEL if CI detected (#22248)

Detect if on CI by checking that the CI environment variable is set to "true".
This is a common pattern among CI providers, including github actions and
cirrus.

.cirrus.yml

@@ -1,7 +1,7 @@
 env:
   CIRRUS_CLONE_DEPTH: '2'
   LANG: en_US.UTF-8
-  CMAKE_EXTRA_FLAGS: -DCI_BUILD=ON -DMIN_LOG_LEVEL=3
+  CMAKE_EXTRA_FLAGS: -DCI_BUILD=ON
 
 freebsd_task:
   name: FreeBSD

.github/workflows/ci.yml

@@ -254,7 +254,7 @@ jobs:
 
       - name: Build
         run: |
-          cmake -B build -G Ninja -D CI_BUILD=ON -D CMAKE_INSTALL_PREFIX:PATH=$INSTALL_PREFIX -D MIN_LOG_LEVEL=3 ${{ matrix.flags }}
+          cmake -B build -G Ninja -D CI_BUILD=ON -D CMAKE_INSTALL_PREFIX:PATH=$INSTALL_PREFIX ${{ matrix.flags }}
           cmake --build build
 
       - if: "!cancelled()"
@@ -383,7 +383,7 @@ jobs:
         run: make deps
 
       - name: Build
-        run: make CMAKE_FLAGS="-D CI_BUILD=ON -D CMAKE_INSTALL_PREFIX:PATH=$INSTALL_PREFIX -D MIN_LOG_LEVEL=3"
+        run: make CMAKE_FLAGS="-D CI_BUILD=ON -D CMAKE_INSTALL_PREFIX:PATH=$INSTALL_PREFIX"
 
       - name: Install
         run: make install

src/nvim/CMakeLists.txt

@@ -412,6 +412,9 @@ else()
 endif()
 
 # Log level (MIN_LOG_LEVEL in log.h)
+if($ENV{CI} MATCHES "true")
+  set(MIN_LOG_LEVEL 3)
+endif()
 if("${MIN_LOG_LEVEL}" MATCHES "^$")
   # Minimize logging for release-type builds.
   target_compile_definitions(main_lib INTERFACE MIN_LOG_LEVEL=$<IF:$<CONFIG:Debug>,1,3>)