new environment variable expansion in rcfile does not extend to command line

the changes from GothenburgBitFactory/libshared#63 pass rcfile values through libshared `Path::expand()`, allowing to expand environment variables:
```
$ echo "rc.alias.foo=$foo >> ~/.taskrc"
$ foo=bar src/task show alias.foo | grep alias
alias.foo bar
```
but this expansion doesn't happen when the values come from the command line instead of rcfile:
```
$ sed -i /alias.foo/d ~/.taskrc
$ foo=bar src/task rc.alias.foo='$foo' show alias.foo | grep alias
alias.foo $foo
```
not sure if this is by design or oversight