ntfy-sh: fix sh not found in subscribe command

ntfy's CLI supports running a command for every incoming message via:

    ntfy subscribe TOPIC COMMAND

See: https://docs.ntfy.sh/subscribe/cli/#run-command-for-every-message

The upstream source hardcodes `sh` as the script launcher in
`cmd/subscribe_unix.go` and `cmd/subscribe_darwin.go`:

```go
scriptLauncher = []string{"sh", "-c"}
```

On NixOS, `sh` is not available in $PATH, causing command execution
to fail with:

    Command failed: exec: "sh": executable file not found in $PATH

Replace the hardcoded `sh` with nixpkgs' runtimeShell, which resolves
to the system's configured shell (typically bash).

pkgs/by-name/nt/ntfy-sh/package.nix

@@ -8,6 +8,7 @@
   mkdocs,
   python3,
   python3Packages,
+  runtimeShell,
 }:
 
 buildGoModule (
@@ -71,6 +72,12 @@ buildGoModule (
 
     postPatch = ''
       sed -i 's# /bin/echo# echo#' Makefile
+      substituteInPlace \
+          cmd/subscribe_unix.go \
+          cmd/subscribe_darwin.go \
+        --replace \
+          'scriptLauncher = []string{"sh", "-c"}' \
+          'scriptLauncher = []string{"${runtimeShell}", "-c"}'
     '';
 
     preBuild = ''