feat: add support for setting build environment variables

[grumpydude]

This adds the capability to pass build time environment variables such as GOOGLE_FUNCTION_SOURCE.

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[matty-rose]

hey @morgante is it possible to tell what the CI failure is?

[morgante]

        	            	FatalError{Underlying: error while running command: exit status 1; ╷
        	            	│ Error: Error waiting for Creating CloudFunctions Function: Error code 13, message: The service has encountered an internal error. Please try again later
        	            	│ 
        	            	│   with module.localhost_function.google_cloudfunctions_function.main,
        	            	│   on ../../main.tf line 90, in resource "google_cloudfunctions_function" "main":
        	            	│   90: resource "google_cloudfunctions_function" "main" {
        	            	│ 
        	            	╵}
        	            	

I'm retriggering the execution.

[bharathkkb]

@morgante I have a fix in 9dea6c8 for build failures. Default SA was disabled which caused CF to fail with a non intuitive error b/227216425

[matty-rose]

Hi @morgante @bharathkkb is this PR able to be merged if the CI failure is unrelated?

[bharathkkb]

Hi @matty-rose Could you rebase this PR?

[grumpydude]

@morgante I've rebased.

[comment-bot-dev]

@grumpydude
Thanks for the PR! 🚀
✅ Lint checks have passed.

[bharathkkb]

Thanks @grumpydude