Give SQLX_OFFLINE_DIR from environment precedence in macros #1421

Job	Run time
Check	1m 46s
Integration Test (macOS-latest)	36s
Integration Test (macOS-13)	1m 59s
Functional Test (MySQL)	59s
Integration Test (ubuntu-latest)	36s
Functional Test (SQLite)	38s
Functional Test (PostgreSQL)	50s
Build (macOS-13)	1m 16s
Build (ubuntu-latest)	51s
Build (windows-latest)	1m 40s
Build (macOS-latest)	42s
	11m 53s

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