build(expo): refactor local build script

[asdolo]

What does this do?

• Update env.js script so it can distinguish if a build is being run locally (with the --local flag).
• Remove build:staging:ios:local and build:staging:android:local scripts from package.json.

Why did you do this?

To avoid having extra scripts when we want to create a local build. We can now do pnpm build:staging:ios --local instead of pnpm build:staging:ios:local.

Who/what does this impact?

Devs using the template.

How did you test this?

Running pnpm build:staging:ios --local and pnpm build:staging:android --local with and without a local .env.staging file and checking that:

1. The environment variables validation script runs. (i.e., it fails if .env.staging is missing, or if a value is missing or have a different type than the expected one).
2. The build succeeds and the built app has the proper values in their environment variables.

[github-actions]

💯 Test Coverage

Lines	Statements	Branches	Functions
	44.6% (223/500)	37.28% (85/228)	30.36% (58/191)

😎 Tests Results

Tests	Skipped	Failures	Errors	Time
53	0 💤	0 ❌	0 🔥	26.871s ⏱️

👀 Tests Details • (45%)

File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
All files	44.6	37.28	30.36	45.89

report-only-changed-files is enabled. No files were changed in this commit :)