nixos/munin: fix broken plugin wrappers

The munin-node service used wrapProgram to inject environment variables.
This doesn't work because munin plugins depend on argv[0], which is
overwritten when the executable is a script with a shebang line (example
below).

This commit removes the wrappers and instead passes the required
environment variables to munin-node.

Eliminating the wrappers resulted in some broken plugins, e.g., meminfo
and hddtemp_smartctl. That was fixed with the per-plugin configuration.

Example:

  The plugin if_eth0 is a symlink to /.../plugins/if_, which uses $0
  to determine that it should monitor traffic on the eth0 interface.

  if_ is a wrapped program, and runs `exec -a "$0" .if_-wrapped`

  .if_-wrapped has a "#!/nix/.../bash" line, which results in bash
  changing $0, and as a result the plugin thinks my interface
  is called "-wrapped".