Fix phx.new crash when parent directory contains a colon

[eagle-head]

mix phx.new crashes with a KeyError when the parent directory
contains a colon (e.g., /mnt/usb-Seagate:0/code/myapp).

The regex in Project.expand_path_with_bindings/2 matches
:[a-zA-Z0-9_]+ against the full absolute path, so a directory
like 0:0 causes :0 to be interpreted as a template binding.

The fix applies the binding expansion only to the relative template
path before joining it with the base path.

To reproduce (before fix)

mkdir 0:0 && cd 0:0
mix phx.new myapp
# => ** (KeyError) key :"0" not found in: %Phx.New.Project{...}

After fix

mkdir 0:0 && cd 0:0
mix phx.new myapp
# => works correctly

Closes #6585