abuild: change $srcdir and $pkgbasedir overrides

Occasionally, a package will use the $srcdir environment variable for
its own purposes and change its behavior during build if it is set.
This is not normally a problem since $srcdir is not normally exported by
abuild. However, if $srcdir is inherited from the environment in order
to override its default value of $startdir/src, it will continue to be
inherited and thus confuse those packages. Instead, check for
$ABUILD_SRCDIR. Likewise for $pkgbasedir and $ABUILD_PKGBASEDIR for
consistency sake.

abuild.in

@@ -2558,8 +2558,8 @@ fi
 APKBUILD=$(readlink -f "$APKBUILD")
 
 startdir="${APKBUILD%/*}"
-srcdir=${srcdir:-"$startdir/src"}
-pkgbasedir=${pkgbasedir:-"$startdir/pkg"}
+srcdir=${ABUILD_SRCDIR:-"$startdir/src"}
+pkgbasedir=${ABUILD_PKGBASEDIR:-"$startdir/pkg"}
 
 repo=${startdir%/*}
 repo=${repo##*/}