Query macros panic if `CARGO` environment variable is not set

[jplatte]

It's not guaranteed that Rust projects are compiled with cargo, so it would probably good to replace this .expect() with a fallback:

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

Line 33 in af7f259

let cargo = env::var_os("CARGO").expect("`CARGO` must be set");

CC rust-lang/rust-analyzer/issues/7462

[abonander]

We also require CARGO_MANIFEST_DIR

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

Line 54 in af7f259

env::var("CARGO_MANIFEST_DIR").map_err(|_| "`CARGO_MANIFEST_DIR` must be set")?;

which is where we look for .env.

I've been told the working dir that proc macros are executed in is always the workspace root, but assuming that is always the case isn't really any better than requiring these env vars to be set.

We need some way to know the directory to search in for sqlx-data.json (or .sqlx/ later on) and .env. Any suggestions?