diff --git a/src/nix/nix-build/nix-build.cc b/src/nix/nix-build/nix-build.cc
index 53dc46eaa6f..4aa6381ee30 100644
--- a/src/nix/nix-build/nix-build.cc
+++ b/src/nix/nix-build/nix-build.cc
@@ -616,7 +616,7 @@ static void main_nix_build(int argc, char ** argv)
         auto tz = getEnv("TZ");
         auto tzExport = tz ? "export TZ=" + escapeShellArgAlways(*tz) + "; " : "";
         std::string rc = fmt(
-                (R"(_nix_shell_clean_tmpdir() { command rm -rf %1%; };)"s
+                ("#!%8%\n" + R"(_nix_shell_clean_tmpdir() { command rm -rf %1%; };)"s
                   "trap _nix_shell_clean_tmpdir EXIT; "
                   "exitHooks+=(_nix_shell_clean_tmpdir); "
                   "failureHooks+=(_nix_shell_clean_tmpdir); ") +
@@ -649,7 +649,7 @@ static void main_nix_build(int argc, char ** argv)
                 escapeShellArgAlways(dirOf(*shell)),
                 escapeShellArgAlways(*shell),
                 tzExport,
-                envCommand);
+                envCommand, *shell);
         vomit("Sourcing nix-shell with file %s and contents:\n%s", rcfile, rc);
         writeFile(rcfile, rc);
 
diff --git a/tests/functional/user-envs.builder.sh b/tests/functional/user-envs.builder.sh
old mode 100644
new mode 100755
index e875c2fe54d..7dd54111ab3
--- a/tests/functional/user-envs.builder.sh
+++ b/tests/functional/user-envs.builder.sh
@@ -1,3 +1,4 @@
+#!/usr/bin/env bash
 # shellcheck shell=bash
 # shellcheck disable=SC2154
 mkdir "$out"