Set CARGO_TARGET_DIR when compiling the prepare queries (#1910)

* Move compiled query data

I did try to set rustc's --out-dir but encountered a strange error
stating that it can not be set more than once (even though I am unable
to deduce what else is setting it).

This enabled me to set a custom CARGO_TARGET_DIR and still be able to
prepare queries.

* Set CARGO_TARGET_DIR in the rustc invocation

sqlx-cli/src/prepare.rs

@@ -172,6 +172,7 @@ hint: This command only works in the manifest directory of a Cargo package."#
             ))
             .env("SQLX_OFFLINE", "false")
             .env("DATABASE_URL", url)
+            .env("CARGO_TARGET_DIR", metadata.target_directory.clone())
             .status()?
     };