Fix build failure on NixOS

[syvb]

Fixes a build error when building on NixOS. This build error was introduced by #31465, which changed filterlock without also updating the vendorTarball hash in etc/shell.nix. (filterlock is not a part of the main Servo workspace (it has its own lockfile and target directory), since it has to be executed before any other Rust code can be compiled on NixOS)

$ nix-shell etc/shell.nix 
...
Running phase: buildPhase
error: failed to select a version for the requirement `toml = "^0.8.9"`
candidate versions found which didn't match: 0.8.8
location searched: directory source `/build/filterlock/vendor` (which is replacing registry `crates-io`)
required by package `filterlock v0.1.0 (/build/filterlock)`
perhaps a crate was updated and forgotten to be re-vendored?
As a reminder, you're using offline mode (--offline) which can sometimes cause surprising resolution failures, if this error is too confusing you may wish to retry without the offline flag.

---

• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes do not require tests because none of the Nix configuration is tested in CI

[delan]

Thanks!