Ensure all mentions of cargo-install are --locked (#21592)

There's already a preference for this in the codebase, but a few of them
have drifted away. Generally `--locked` is preferred to reduce exposure
to supply-chain attacks (and just generally improve reproducibility).

In an ideal world these dependencies would maybe even be pinned to
versions but Cargo is kinda bad at that for devtools. Still better to
use --locked than not.

AGENTS.md

@@ -130,7 +130,7 @@ When UI or text output changes intentionally, update the snapshots as follows:
 
 If you don’t have the tool:
 
-- `cargo install cargo-insta`
+- `cargo install --locked cargo-insta`
 
 ### Test assertions