tidy-up: use /usr/bin/env perl shebang

Most Perl scripts already used it. Sync up the few outliers.

Closes #14029

.github/scripts/badwords.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 # Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
 #
 # SPDX-License-Identifier: curl

.github/scripts/cleancmd.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 # Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
 #
 # SPDX-License-Identifier: curl

.github/scripts/cleanspell.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 # Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
 #
 # SPDX-License-Identifier: curl

scripts/ciconfig.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 #***************************************************************************
 #                                  _   _ ____  _
 #  Project                     ___| | | |  _ \| |

scripts/cijobs.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 #***************************************************************************
 #                                  _   _ ____  _
 #  Project                     ___| | | |  _ \| |