Display valid installable in InstallableDerivedPath::parse warning

The warning message should produce an installable name that can be
passed to `nix build`, `nix path-info`, etc. again. Since the CLI
expects that the .drv path and the output names are separated by
a caret, the warning message must also separate the .drv path and output
names with a caret.

However, `DerivedPath::Built.to_string()` uses an exclamation point as
the separator instead. This commit adds a `separator` argument to the
to_string method.

This changes the warning message from:
If this command is now failing try again with '/nix/store/foo.drv!*'
to:
If this command is now failing try again with '/nix/store/foo.drv^*'

0 files changed, 0 insertions, 0 deletions