Delete build directory at the start of a dispatched build.

Previously, if a dispatched build failed the files from the previous
build would still get copied.  This was an intentional feature, since
only the compiled binaries of these builds are used, and usually these
would work fine (with the zo files that were built on the main machine).

Now, the build directory is deleted when starting, which means that any
failure in a remote build means that the whole build will fail.  This
turns out to be better since when there are problems, the previous
behavior could be very confusing.  In addition, the racket setup itself
rarely fails, so this won't make a difference.

collects/meta/build/build

@@ -1154,7 +1154,7 @@ DO_COPY_BUILD() { # inputs -- machine-name (for ssh)
   if [[ "$machine" != "$workmachine" ]]; then
     show "Running DO_BUILD on $machine in the background"
     _run ssh "$machine" \
-      "mkdir -p \"$tmpdir\"; cat > \"$repotgz\"" \
+      "rm -rf \"$tmpdir\"; mkdir -p \"$tmpdir\"; cat > \"$repotgz\"" \
       < "$repotgz"
     touch "$bglogfile-$machine"
     chcon --type=httpd_sys_content_t "$bglogfile-$machine"