Get rid of CanonPath::fromCwd

As discussed in the last Nix team meeting (2024-02-95), this method
doesn't belong because `CanonPath` is a virtual/ideal absolute path
format, not used in file systems beyond the native OS format for which a
"current working directory" is defined.

Progress towards #9205

src/libcmd/repl.cc

@@ -899,7 +899,7 @@ void NixRepl::addVarToScope(const Symbol name, Value & v)
 
 Expr * NixRepl::parseString(std::string s)
 {
-    return state->parseExprFromString(std::move(s), state->rootPath(CanonPath::fromCwd()), staticEnv);
+    return state->parseExprFromString(std::move(s), state->rootPath("."), staticEnv);
 }