build: remove Bash dependency in favor of sh

These shell commands/scripts all seem to work fine with POSIX shell, so
there's no reason to depend on Bash for building.

doc/dbus/meson.build

@@ -18,7 +18,7 @@ foreach interface_file: dbus_interfaces
         depend_files: interface_file,
         capture: true,
         command: [
-            bash, '-c',
+            sh, '-c',
             'cat "$1"; "$2" "$3" "$4" | tail -n +2',
             '_', # argv0 ignored
             '@INPUT@',

po/check-translations.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 po_dir=$(dirname $0)
 

src/meson.build

@@ -1,4 +1,4 @@
-bash = find_program('bash')
+sh = find_program('sh')
 dbus_interfaces = files(
     'net.reactivated.Fprint.Manager.xml',
     'net.reactivated.Fprint.Device.xml',