cmake: Fix potential configure error when setting up buildnumber

If the "CI" environment variable is defined but "GITHUB_RUN_ID" is not
this will result in a broken if statement.
For example, this can be the case for some Debian build pipelines

cmake/common/buildnumber.cmake

@@ -12,7 +12,7 @@ if(NOT DEFINED PLUGIN_BUILD_NUMBER AND EXISTS "${_BUILD_NUMBER_CACHE}")
   file(READ "${_BUILD_NUMBER_CACHE}" PLUGIN_BUILD_NUMBER)
   math(EXPR PLUGIN_BUILD_NUMBER "${PLUGIN_BUILD_NUMBER}+1")
 elseif(NOT DEFINED PLUGIN_BUILD_NUMBER)
-  if($ENV{CI} AND $ENV{GITHUB_RUN_ID})
+  if("$ENV{CI}" AND "$ENV{GITHUB_RUN_ID}")
     set(PLUGIN_BUILD_NUMBER "$ENV{GITHUB_RUN_ID}")
   else()
     set(PLUGIN_BUILD_NUMBER "1")