build: nix flake and Cargo dependency updates (#3)

- Changes inspired by
https://fasterthanli.me/series/building-a-rust-service-with-nix/: sentry
- move dependencies to workspace
- `agg` now is available in `nixpkgs`, add it to dev shell to make it
easier to create demo GIFs =]

.envrc

@@ -1 +1,3 @@
 use flake
+
+dotenv .env

.github/workflows/ci.yml

@@ -13,17 +13,12 @@ jobs:
       # Allow other jobs to finish building and cache properly before bailing
       fail-fast: false
       matrix:
-        include:
-          - os: ubuntu-latest
-            # Latest and greatest release of Nix
-            install_url: https://nixos.org/nix/install
+        os: [ubuntu-latest, macos-latest]
 
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v3
     - uses: cachix/install-nix-action@v20
-      with:
-        install_url: ${{ matrix.install_url }}
     - uses: cachix/cachix-action@v12
       with:
         name: sourmash-bio

.gitignore

@@ -1,2 +1,4 @@
 /target
 .direnv/
+sandbox/
+result