source: fix variable interpretation

[flatcap]

Change source to look for NeoMutt variables first.

---

The source and set commands differed in how they interpreted a
$variable name.

set my_c = `echo $my_dir/test.rc`

set would look for a NeoMutt variable called my_dir and failing that
look for an extern environment variable with that name.

source `echo $my_dir/test.rc`

source only looked for an extern environment variable.