Write the apps snapshot in config init only on macOS (closes #408)

`config init` wrote an `apps` snapshot of `/Applications` on every
system. That directory exists only on macOS, so on Linux the first
`snapshot create` stopped on it unless `--skip-errors` was given.

`defaultConfigText` takes the operating system name and returns the
file with the `apps` snapshot and its comment only for `darwin`;
`config init` passes `runtime.GOOS`. The macOS file is unchanged byte
for byte.

Model: opus-5-5

README.md

@@ -32,8 +32,8 @@ mkdir /Volumes/usbstick/mybackup
 # (System Settings → Privacy & Security → Full Disk Access), otherwise
 # the backup will abort with a permission error on protected directories
 
-# run your first backup (the default config backs up ~ and /Applications
-# with sensible excludes)
+# run your first backup (the default config backs up ~, and /Applications
+# on macOS only, with sensible excludes)
 vaultik snapshot create
 
 # see what you have