Just a style issue; use `export` instead of `unset` like other variables

plugins/python-build/test/cache.bats

@@ -3,7 +3,7 @@
 load test_helper
 export PYTHON_BUILD_SKIP_MIRROR=1
 export PYTHON_BUILD_CACHE_PATH="$TMP/cache"
-unset PYTHON_BUILD_ARIA2_OPTS
+export PYTHON_BUILD_ARIA2_OPTS=
 
 setup() {
   mkdir "$PYTHON_BUILD_CACHE_PATH"

plugins/python-build/test/checksum.bats

@@ -3,7 +3,7 @@
 load test_helper
 export PYTHON_BUILD_SKIP_MIRROR=1
 export PYTHON_BUILD_CACHE_PATH=
-unset PYTHON_BUILD_ARIA2_OPTS
+export PYTHON_BUILD_ARIA2_OPTS=
 
 
 @test "package URL without checksum" {

plugins/python-build/test/mirror.bats

@@ -4,7 +4,7 @@ load test_helper
 export PYTHON_BUILD_SKIP_MIRROR=
 export PYTHON_BUILD_CACHE_PATH=
 export PYTHON_BUILD_MIRROR_URL=http://mirror.example.com
-unset PYTHON_BUILD_ARIA2_OPTS
+export PYTHON_BUILD_ARIA2_OPTS=
 
 
 @test "package URL without checksum bypasses mirror" {