macros-core: give SQLX_OFFLINE_DIR from environment precedence (#3962)

We need to be able to supply this variable via the environment when built via
Nix Crane, which uses cargo vendor, which will filter the .sqlx path.  Renaming
is necessary in that case.

Author: psionic-k

sqlx-macros-core/src/query/mod.rs

@@ -124,6 +124,8 @@ fn init_metadata(manifest_dir: &String) -> crate::Result<Metadata> {
         .map(|s| s.eq_ignore_ascii_case("true") || s == "1")
         .unwrap_or(false);
 
+    let offline_dir = env("SQLX_OFFLINE_DIR").ok().or(offline_dir);
+
     let config = Config::try_from_crate_or_default()?;
 
     let database_url = env(config.common.database_url_var()).ok().or(database_url);