fish: fix postrm script interpreter

[dhrm1k]

Add the missing shell interpreter line to the fish postrm script so apk can
run the generated post-deinstall hook when fish is removed.

Without the shebang, apk:

fish-4.1.2-r2.post-deinstall: execve: Exec format error

Fixes: #29348

📦 Package Details

Maintainer: Curtis Jiang @jqqqqqqqqqq

Description:
Fix the fish package removal hook by adding the missing #!/bin/sh line to
the generated postrm script.

---

🧪 Run Testing Details

• OpenWrt Version: 25.12.4
• OpenWrt Target/Subtarget: bcm53xx/generic
• OpenWrt Device: asus rt ac68u

Tested with:

apk add fish
apk del fish

Before this change, apk del fish reported execve: Exec format error while
running the fish post-deinstall hook.

---

✅ Formalities

• I have reviewed the CONTRIBUTING.md file for detailed contributing guidelines.

If your PR contains a patch:

• Not applicable, this PR does not add or modify an upstream source patch.

[dhrm1k]

CI is failing.
pull_request_target workflow refuses to check out fork PR code.

My previous PRs did not hit this, Did it change recently?

[dhrm1k]

I noticed this same apk-side failure in another package too (arp-scan-database).

It looks like the common pattern is that apk executes the generated maintainer
hook directly, so if the generated postrm / postinst script does not start
with something like #!/bin/sh, it fails with execve: Exec format error.

Will it be helpful if I look into a few more of these, or will it be considered
spamming?