libexec/build.sh: set XBPS_ARCH while installing base-files to the masterdir.

Fixes './xbps-src bootstrap' in musl hosts.

common/xbps-src/libexec/build.sh

@@ -103,7 +103,7 @@ if [ -z "$CHROOT_READY" -a "$PKGNAME" = "base-files" ]; then
     if [ -n "$XBPS_BUILD_FORCEMODE" ]; then
         _flags="-f"
     fi
-    $XBPS_INSTALL_CMD ${_flags} -y $PKGNAME >${_log} 2>&1
+    XBPS_ARCH=$XBPS_MACHINE $XBPS_INSTALL_CMD ${_flags} -y $PKGNAME >${_log} 2>&1
     if [ $? -ne 0 ]; then
         msg_red "Failed to install $PKGNAME into masterdir, see below for errors:\n"
         cat ${_log}