ci: always run cachix on master push (#14)

## Summary
- Drops the `paths:` filter on `.github/workflows/cachix.yaml` so every
push to `master` builds and pushes a binary to the abizer cachix cache.
- The Nix derivation pins `version = self.shortRev` and `src = self`, so
every commit on master produces a fresh store-path hash — including the
ones the previous filter skipped (release workflows, brew bottle
updates, README/doc-only changes).
- That mismatch is what just stranded master at 6deeb98 with cachix
still pointing at bfa3180; flake users were left building from source.

## Why drop the filter instead of widening it
- Cachix builds finish in ~2m per system, and the matrix is just
`x86_64-linux` + `aarch64-darwin`.
- Maintaining a list of "things that go into the derivation" means
tracking flake.nix inputs forever; one easier rule wins over a clever
one that keeps drifting.

## Test plan
- [ ] After merge, confirm the cachix run for the merge commit completes
and the new short-rev store path is fetchable from `abizer.cachix.org`.
- [ ] `nix build .#packages.aarch64-darwin.default` on the new rev
should be a pure cache fetch (no local compile).

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

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk: this only changes GitHub Actions trigger conditions, but it
may increase CI workload/cost by running on every `master` push.
> 
> **Overview**
> Removes the `paths:` filter from `.github/workflows/cachix.yaml` so
the Cachix build runs on **every** push to `master` (and via manual
dispatch), rather than only when certain files change.
> 
> This ensures the Nix build is executed and pushed to cache for all
commits, including doc-only or release-related changes that previously
skipped the workflow.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
504eca8. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: abizer

.github/workflows/cachix.yaml

@@ -3,12 +3,6 @@ name: cachix
 on:
   push:
     branches: [master]
-    paths:
-      - "cmd/nssh/**"
-      - "internal/**"
-      - ".github/workflows/cachix.yaml"
-      - "go.*"
-      - "flake.*"
   workflow_dispatch:
 
 jobs: