start-container: Change workdir to match ocivm

The `ocivm.sh` script mounts the source directory at `/tmp/src`, since
we use `/tmp/build` as the output directory.  For consistency, the
`start-container.sh` script, although it's mostly unused anymore, now
does the same.

start-container.sh

@@ -13,8 +13,8 @@ podman run \
     -v distfiles:/var/cache/distfiles \
     -v packages:/usr/${target}/var/cache/binpkgs \
     -v /mnt/gentoo \
-    -v ${PWD}:/tmp/build:rw,z \
-    -w /tmp/build \
+    -v ${PWD}:/tmp/src:rw,z \
+    -w /tmp/src \
     -d \
     -e FEATURES='-ipc-sandbox -network-sandbox -pid-sandbox' \
     cross-${target} \