Add /bin/sh to bubblewrap sandbox

I ran into this error when trying to run `opendkim-genkey` in a vars
generator:

```console
=========================================================================== Command ===========================================================================
nix \
    --extra-experimental-features 'nix-command flakes' \
    shell \
    --inputs-from /nix/store/9r3ddw80dz4qzci9pj57ppbh6gy2pgv9-clan-cli/lib/python3.12/site-packages/clan_cli/nixpkgs \
    'nixpkgs#bash' \
    'nixpkgs#bubblewrap' \
    -c bwrap \
    --unshare-all --tmpfs \
    / \
    --ro-bind /nix/store \
    /nix/store \
    --dev /dev \
    --bind /tmp/nix-shell.ClOjgJ/vars-kh4qrnas \
    /tmp/nix-shell.ClOjgJ/vars-kh4qrnas \
    --chdir / \
    --bind /proc \
    /proc \
    --uid 1000 \
    --gid 1000 \
    -- bash \
    -c /nix/store/p0089w4y1w3h535g7ipv4jl4r6mb2hs2-generator-dkim-playground.jflei.com.mail

=========================================================================== Stderr ============================================================================
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
	LANGUAGE = (unset),
	LC_ALL = (unset),
	LC_CTYPE = (unset),
	LC_NUMERIC = (unset),
	LC_COLLATE = (unset),
	LC_TIME = (unset),
	LC_MESSAGES = (unset),
	LC_MONETARY = (unset),
	LC_ADDRESS = (unset),
	LC_IDENTIFICATION = (unset),
	LC_MEASUREMENT = (unset),
	LC_PAPER = (unset),
	LC_TELEPHONE = (unset),
	LC_NAME = (unset),
	LANG = "en_US.UTF-8"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").
Can't exec "/bin/sh": No such file or directory at /nix/store/nfawbww80p1hgpymfgq1vq8wqlak75yh-opendkim-2.11.0-Beta2/sbin/.opendkim-genkey-wrapped line 139.
.opendkim-genkey-wrapped: openssl died with signal %d
127
Return Code: 1

1 hosts failed with an error. Check the logs above
```

As we allow `/bin/sh` in the nix build sandbox, I assume we're OK
allowing it here as well?

pkgs/clan-cli/clan_cli/facts/generate.py

@@ -48,6 +48,7 @@ def bubblewrap_cmd(generator: str, facts_dir: Path, secrets_dir: Path) -> list[s
             "--unshare-all",
             "--tmpfs",  "/",
             "--ro-bind", "/nix/store", "/nix/store",
+            "--ro-bind", "/bin/sh", "/bin/sh",
             "--dev", "/dev",
             # not allowed to bind procfs in some sandboxes
             "--bind", str(facts_dir), str(facts_dir),

pkgs/clan-cli/clan_cli/vars/generate.py

@@ -103,6 +103,7 @@ def bubblewrap_cmd(generator: str, tmpdir: Path) -> list[str]:
             "--unshare-all",
             "--tmpfs",  "/",
             "--ro-bind", "/nix/store", "/nix/store",
+            "--ro-bind", "/bin/sh", "/bin/sh",
             *(["--ro-bind", str(test_store), str(test_store)] if test_store else []),
             "--dev", "/dev",
             # not allowed to bind procfs in some sandboxes