Bug 1959469 - Fix ./mach uniffi generate, r=markh,glandium

The issue was that the `default-hasher` feature wasn't turned on, which
made it so the build hack that substitutes older hashbrown versions with
v0.15.2 was failing.

Differential Revision: https://phabricator.services.mozilla.com/D244942

build/rust/hashbrown/Cargo.toml

@@ -9,7 +9,12 @@ path = "lib.rs"
 [features]
 default = ["hashbrown/default"]
 allocator-api2 = ["hashbrown/allocator-api2"]
-ahash = [] # No direct equivalent in 0.15.*.
+ahash = [
+    # No direct equivalent in 0.15.*.  However, the `ahash` feature made it so `DefaultHashBuilder`
+    # was a real struct that implemented `BuildHasher`, `Default`, etc.
+    # Enable 0.15.* `default-hasher` feature, which does the same.
+    "hashbrown/default-hasher"
+]
 inline-more = ["hashbrown/inline-more"]
 raw = ["hashbrown/raw-entry"]
 serde = ["hashbrown/serde"]