abuild-rootbld: clear environment for bwrap container

Otherwise, user-set environment variables can leak into the container
and cause spurious build/test failures. A common example is the value of
the SHELL environment variable which is used by a lot of software.
Outside of the bwrap container I use ksh and my SHELL environment
variable points to /bin/ksh, however, inside the container /bin/ksh is
not available and hence software relying on $SHELL doesn't work
properly. This can cause annoying to debug test failures, e.g. !43430.

abuild.in

@@ -2508,6 +2508,10 @@ rootbld() {
 		--bind "$REPODEST" "$REPODEST" \
 		--hostname "build-$buildhost-$CARCH" \
 		--chdir "$startdir" \
+		--clearenv \
+		--setenv HOME "$HOME" \
+		--setenv SOURCE_DATE_EPOCH "$SOURCE_DATE_EPOCH" \
+		--setenv ABUILD_LAST_COMMIT "$ABUILD_LAST_COMMIT" \
 		--setenv PATH ${USE_CCACHE:+/usr/lib/ccache/bin:}/bin:/usr/bin:/sbin:/usr/sbin \
 		--setenv FAKEROOTDONTTRYCHOWN 1 \
 		--unsetenv CBUILD \