guix: Construct $OUTDIR in ${DISTSRC}/output

While files are being output to $OUTDIR, it will be under
${DISTSRC}/output, and only when everything is done, will
${DISTSRC}/output be moved to the actual $OUTDIR.

This makes it so that a Ctrl-C in the middle of a build is less likely
to result in a partially-constructed $OUTDIR. In fact, if I understand
correctly, if $OUTDIR and $DISTSRC reside on the same filesystem, the
move (rename) is likely atomic.

Also, since the "working $OUTDIR" is under ${DISTSRC}/output, it will be
cleaned properly by the guix-clean script.

contrib/guix/libexec/build.sh

@@ -33,6 +33,9 @@ Required environment variables as seen inside the container:
     OUTDIR: ${OUTDIR:?not set}
 EOF
 
+ACTUAL_OUTDIR="${OUTDIR}"
+OUTDIR="${DISTSRC}/output"
+
 #####################
 # Environment Setup #
 #####################
@@ -429,3 +432,5 @@ mkdir -p "$DISTSRC"
             ;;
     esac
 )  # $DISTSRC
+
+mv --no-target-directory "$OUTDIR" "$ACTUAL_OUTDIR"