Fix functional tests failing under nextest (mitsuhiko#824)

## Summary

- Fix `test_pending_snapshot_deletion` failing when run under nextest
- Clear `NEXTEST_RUN_ID` environment variable in functional test setup

When running functional tests under nextest, the `NEXTEST_RUN_ID`
environment variable was inherited by child `cargo insta test`
invocations. Since insta uses this variable as the `RUN_ID` for pending
snapshots (see `insta/src/snapshot.rs:18`), all pending inline snapshots
ended up sharing the same run_id.

This broke the cleanup mechanism in `load_batch()` which keeps all
entries with the last run_id. When a test passed and wrote a NULL entry
to mark the snapshot as clean, the previous failing entry was also kept
(same run_id), preventing the `.pending-snap` file from being deleted.

## Test plan

- [x] `cargo test --package cargo-insta` passes
- [x] `cargo nextest run --package cargo-insta` passes (previously
failed on `test_pending_snapshot_deletion`)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: max-sixty

cargo-insta/tests/functional/main.rs

@@ -216,6 +216,9 @@ impl TestProject {
         cmd.env_remove("CARGO_TERM_COLOR");
         cmd.env_remove("CLICOLOR_FORCE");
         cmd.env_remove("RUSTDOCFLAGS");
+        // Remove NEXTEST_RUN_ID so that each cargo insta test invocation gets
+        // its own unique run_id, rather than all sharing the outer nextest's ID
+        cmd.env_remove("NEXTEST_RUN_ID");
     }
 
     fn insta_cmd(&self) -> Command {