Faster cold local test builds with single-locale compilation

The test environment now compiles only the English locale by default, matching the dev environment, so a fresh worktree’s first cold mix test no longer regenerates all ten ex_cldr locales. The TUIST_DEV_ALL_LOCALES=1 flag still compiles every locale, and CI continues to use that flag so internationalization tests remain covered. No production behaviour is changed: managed prod builds still compile all locales.