Auto merge of #8421 - alexcrichton:read-env-dep, r=ehuss

Parse `# env-dep` directives in dep-info files

This commit updates Cargo's parsing of rustc's dep-info files to account
for changes made upstream in rust-lang/rust#71858. This means that if
`env!` or `option_env!` is used in crate files Cargo will correctly
rebuild the crate if the env var changes.

Closes #8417

src/cargo/core/compiler/context/compilation_files.rs

@@ -490,7 +490,7 @@ fn compute_metadata(
     // cause a new hash due to the rustc version changing, but this allows
     // cargo to be extra careful to deal with different versions of cargo that
     // use the same rustc version.
-    1.hash(&mut hasher);
+    2.hash(&mut hasher);
 
     // Unique metadata per (name, source, version) triple. This'll allow us
     // to pull crates from anywhere without worrying about conflicts.