fix: work inside a flake

While we don't want to encourage it, it would be helpful to ease
adoption if users could call this module function from inside their
flake.nix. The only thing preventing that was the path referencing
becoming a pure /nix/store path reference implicitly (via the flake)
and so failing because of the string context.

Fix this by simply removing the string context. This isn't actually
unsafe since if the absolute path literal refers to a store path and is
ever coerced back to a string, nix will readd the context automatically.

std/path/strToPath.nix

@@ -1,7 +1,10 @@
 str:
 let
   # strip off the first `/` of an absolute path
-  frag = std.substring 1 (-1) str;
+  # disgarding the string context of a store path should be safe here
+  # since if the path refers to a store path, the context will be readded
+  # if/when the path is turned back into a string
+  frag = std.unsafeDiscardStringContext (std.substring 1 (-1) str);
   # will fail if the string does not represent an absolute path
   # which is what we want since this function makes little sense otherwise
   validate = std.toPath str;