Fix devShell handling of env values including @ and %

src/nix/develop.cc

@@ -54,7 +54,7 @@ BuildEnvironment readEnvironment(const Path & path)
         R"re((?:[a-zA-Z_][a-zA-Z0-9_]*))re";
 
     static std::string simpleStringRegex =
-        R"re((?:[a-zA-Z0-9_/:\.\-\+=]*))re";
+        R"re((?:[a-zA-Z0-9_/:\.\-\+=@%]*))re";
 
     static std::string dquotedStringRegex =
         R"re((?:\$?"(?:[^"\\]|\\[$`"\\\n])*"))re";

tests/shell.nix

@@ -34,6 +34,8 @@ let pkgs = rec {
     name = "shellDrv";
     builder = "/does/not/exist";
     VAR_FROM_NIX = "bar";
+    ASCII_PERCENT = "%";
+    ASCII_AT = "@";
     TEST_inNixShell = if inNixShell then "true" else "false";
     inherit stdenv;
     outputs = ["dev" "out"];