Use find_program to get python in cmake (#546)

* Use find_program to get python in cmake

* Update build_linux.yml

* Revert "Update build_linux.yml"

This reverts commit 82bd9f4.

Author: a-maurice

CMakeLists.txt

@@ -146,6 +146,12 @@ else()
   set(DESKTOP OFF)
 endif()
 
+# Define the Python executable before including the subprojects
+find_program(FIREBASE_PYTHON_EXECUTABLE
+  NAMES python3 python
+  REQUIRED
+)
+
 include(FindPkgConfig)
 include(android_dependencies)
 include(build_universal)

cmake/build_aar.cmake

@@ -45,7 +45,7 @@ function(build_aar LIBRARY_NAME LIBRARY_TARGET PROGUARD_TARGET
 
   add_custom_command(
     OUTPUT "${OUTPUT_AAR}"
-    COMMAND python "${FIREBASE_SOURCE_DIR}/aar_builder/build_aar.py"
+    COMMAND ${FIREBASE_PYTHON_EXECUTABLE} "${FIREBASE_SOURCE_DIR}/aar_builder/build_aar.py"
       "--output_file=${OUTPUT_AAR}"
       "--library_file=$<TARGET_FILE:${LIBRARY_TARGET}>"
       "--architecture=${ANDROID_ABI}"

cmake/firebase_swig.cmake

@@ -202,13 +202,13 @@ macro(firebase_swig_add_library name)
     OUTPUT ${UNITY_SWIG_CS_FIX_FILE}
     DEPENDS ${UNITY_SWIG_CS_GEN_FILE}
     COMMAND
-      python
+      ${FIREBASE_PYTHON_EXECUTABLE}
         ${CMAKE_CURRENT_LIST_DIR}/../swig_commenter.py
         --input=\"${all_cpp_header_files}\"
         --output=\"${UNITY_SWIG_CS_GEN_FILE}\"
         --namespace_prefix=\"Firebase\"
     COMMAND
-      python
+      ${FIREBASE_PYTHON_EXECUTABLE}
         ${FIREBASE_SWIG_FIX_PY}
         --language=csharp
         --in_file=\"${UNITY_SWIG_CS_GEN_FILE}\"
@@ -233,7 +233,7 @@ macro(firebase_swig_add_library name)
     OUTPUT ${UNITY_SWIG_CPP_FIX_FILE}
     DEPENDS ${UNITY_SWIG_CPP_GEN_FILE}
     COMMAND
-      python
+      ${FIREBASE_PYTHON_EXECUTABLE}
         ${FIREBASE_SWIG_FIX_PY}
         --language=cpp
         --in_file=\"${UNITY_SWIG_CPP_GEN_FILE}\"