abuild: do not export "$srcdir"

several[1][2] build systems use a variable called "srcdir"
to locate either the root project directory, or some subdirectory

we should not blindly export a variable under the same name,
as it leads to broken builds - either loudly, when tests fail,
or quietly when a ""missing"" file simply gets ignored

[1]: https://github.com/PCRE2Project/pcre2/blob/pcre2-10.45/RunTest#L141
[2]: https://github.com/json-c/json-c/blob/json-c-0.18/tests/test-defs.sh#12

abuild.in

@@ -2614,7 +2614,7 @@ rootbld() {
 		--hostname "build-$buildhost-$CARCH" \
 		--chdir "$startdir" \
 		--clearenv \
-		--setenv srcdir /tmp/src \
+		--setenv abuild_srcdir /tmp/src \
 		--setenv pkgbasedir /tmp/pkg \
 		--setenv tmpdir /tmp/tmp \
 		--setenv CARCH "$CARCH" \
@@ -2937,7 +2937,7 @@ fi
 APKBUILD=$(readlink -f "$APKBUILD")
 
 startdir="${APKBUILD%/*}"
-srcdir=${srcdir:-"$startdir/src"}
+srcdir=${srcdir:-${abuild_srcdir:-"$startdir/src"}}
 pkgbasedir=${pkgbasedir:-"$startdir/pkg"}
 tmpdir=${tmpdir:-"$startdir/tmp"}