Evaluate nix-shell -i args relative to script

When writing a shebang script, you expect your path to be relative to
the script, not the cwd. We previously handled this correctly for
relative file paths, but not for expressions.

This handles both -p & -E args. My understanding is this should be
what we want in any cases I can think of - people run scripts from
many different working directories. @edolstra is there any reason to
handle -p args differently in this case?

Fixes NixOS#4232

src/nix-build/nix-build.cc

@@ -298,7 +298,9 @@ static void main_nix_build(int argc, char * * argv)
     else
         for (auto i : left) {
             if (fromArgs)
-                exprs.push_back(state->parseExprFromString(std::move(i), state->rootPath(CanonPath::fromCwd())));
+                exprs.push_back(state->parseExprFromString(
+                    std::move(i),
+                    state->rootPath(CanonPath::fromCwd(inShebang ? dirOf(script) : "."))));
             else {
                 auto absolute = i;
                 try {
@@ -311,7 +313,7 @@ static void main_nix_build(int argc, char * * argv)
                     /* If we're in a #! script, interpret filenames
                        relative to the script. */
                     exprs.push_back(state->parseExprFromFile(resolveExprPath(state->checkSourcePath(lookupFileArg(*state,
-                                        inShebang && !packages ? absPath(i, absPath(dirOf(script))) : i)))));
+                                        inShebang ? absPath(i, absPath(dirOf(script))) : i)))));
             }
         }