UnquotedVariable: fix false positives with unset_command

Caught on sci-biology/hmmer, it was incorrectly reporting for TMPDIR
declared with `unset`.

Signed-off-by: Arthur Zamarin <arthurzam@gentoo.org>

Author: arthurzam

src/pkgcheck/checks/codingstyle.py

@@ -1019,6 +1019,8 @@ class _UnquotedVariablesCheck(Check):
         # Variable is part of a shell assignment, and does not need to be
         # quoted. for example S=${WORKDIR}/${PN} is ok.
         'variable_assignment',
+        # Variable is being used in a unset command.
+        'unset_command',
         # Variable is part of declaring variables, and does not need to be
         # quoted. for example local TMPDIR is ok.
         'declaration_command',

testdata/repos/standalone/EbuildUnquotedVariablesCheck/EbuildUnquotedVariable/EbuildUnquotedVariable-0.ebuild

@@ -44,4 +44,6 @@ src_prepare() {
 	local TMPDIR # ok
 	TMPDIR="$(mktemp --directory --tmpdir=/tmp ${PN}-XXXX)" # ok
 	export TMPDIR # ok
+
+	unset TMPDIR # ok
 }