Fix Dragonfly BSD CI

Dragonfly BSD 6.4.0 VM doesn't seem to have git.

```
  error: failed to get `cfg-if` as a dependency of package `polling v2.5.2 (/Users/runner/work/polling/polling)`

  Caused by:
    failed to load source for dependency `cfg-if`

  Caused by:
    Unable to update registry `crates-io`

  Caused by:
    failed to fetch `https://github.com/rust-lang/crates.io-index`

  Caused by:
    could not execute process `git fetch --force --update-head-ok 'https://github.com/rust-lang/crates.io-index' '+HEAD:refs/remotes/origin/HEAD'` (never executed)

  Caused by:
    No such file or directory (os error 2)
```

.github/workflows/ci.yml

@@ -145,7 +145,7 @@ jobs:
         usesh: true
         # Dragonfly BSD is tier 3 target, so install rust from package manager instead of rustup
         prepare: |
-          pkg install -y rust
+          pkg install -y git rust
         run: |
           cargo test