build: fix build labels on GitHub Actions

CMakeLists.txt

@@ -14,6 +14,17 @@ execute_process(
   OUTPUT_VARIABLE GIT_DESCRIBE
   OUTPUT_STRIP_TRAILING_WHITESPACE
 )
+message(STATUS "git describe returned \"${GIT_DESCRIBE}\"")
+
+# git describe fails on GitHub Actions
+if(NOT GIT_DESCRIBE)
+	# See: https://docs.github.com/en/actions/reference/environment-variables#default-environment-variables
+	if(NOT ("$ENV{GITHUB_REF_NAME}" STREQUAL ""))
+		set(GIT_DESCRIBE "$ENV{GITHUB_REF_NAME}")
+
+		message(STATUS "using version label \"${GIT_DESCRIBE}\" from GITHUB_REF_NAME")
+	endif()
+endif()
 
 if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND CMAKE_CXX_SIMULATE_ID STREQUAL "MSVC")
 	add_compile_options(/EHsc)