abump: demonstrate abump environment polution

abump sources the APKBUILD to be able to check some variables. When the
APKBUILD exports variables in the global scope, that affects the abump
environment as well.

When abump then executes abuild, it will inherrit the environment from
abump. This is an issue under the following circumstances:

* The APKBUILD only updates the value of an exported variable if it's
  not set
* The default value includes a variable set by abuild, like `$srcdir`.

Because the variable is set by abuild, but not abump, the resulting
exported variable is different. Because it's then set incorrectly in the
abump environment, it's no longer updated with the correct variable when
abuild is invoked.

tests/abump_test

@@ -5,7 +5,8 @@ init_tests \
 	abump_help \
 	abump_invalid_opt \
 	abump_missing_args \
-	abump_simple_bump
+	abump_simple_bump \
+	abump_isolates_apkbuild
 
 export ABUILD_SHAREDIR=$(atf_get_srcdir)/..
 export GIT_CONFIG_GLOBAL="$(atf_get_srcdir)/testdata/gitconfig"
@@ -76,3 +77,39 @@ abump_simple_bump_body() {
 		abump foo-1.2
 }
 
+abump_isolates_apkbuild_body() {
+	cp -r "$(atf_get_srcdir)"/testdata/.abuild .
+	git init
+	mkdir -p main/bar
+	cd main/bar
+	echo "first" > bar-1.0.txt
+	cat > APKBUILD <<-"EOF"
+		# Maintainer: Test user <user@example.com>
+		pkgname="bar"
+		pkgver=1.0
+		pkgrel=0
+		pkgdesc="dummy package for test"
+		url="https://alpinelinux.org"
+		license="MIT"
+		arch="noarch"
+		source="bar-$pkgver.txt"
+		options="!check"
+
+		export BUILDFLAGS="${BUILDFLAGS:-"repo=$repo"}"
+
+		package() {
+			echo "BUILDFLAGS: $BUILDFLAGS"
+			install -D "$srcdir"/bar-$pkgver.txt "$pkgdir"/bar
+		}
+	EOF
+	abuild checksum
+	abuild
+	git add APKBUILD bar-1.0.txt
+	git commit -m "test commit"
+
+	echo "second" > bar-1.1.txt
+	atf_check \
+		-o match:"BUILDFLAGS: repo=main" \
+		-e ignore \
+		abump bar-1.1
+}