Auto merge of #118830 - GuillaumeGomez:env-tracked_env, r=Nilstrieb

Add support for `--env` on `tracked_env::var`

Follow-up of rust-lang/rust#118368.
Part of Part of rust-lang/rust#80792.

It adds support of the `--env` option for proc-macros through `tracked_env::var`.

r? `@Nilstrieb`

crates/proc-macro-srv/src/server.rs

@@ -58,6 +58,10 @@ impl server::Types for RustAnalyzer {
 }
 
 impl server::FreeFunctions for RustAnalyzer {
+    fn injected_env_var(&mut self, _var: &str) -> Option<String> {
+        None
+    }
+
     fn track_env_var(&mut self, _var: &str, _value: Option<&str>) {
         // FIXME: track env var accesses
         // https://github.com/rust-lang/rust/pull/71858