test(tvix/[ca]store): enable bigtable only with "integration" feature

The emulator and bigtable client are quite big. Remove them from the
default //tvix:shell.

Put the tests behind a `integration` feature flag, and add a variant
with that enabled to CI, and drop the bigtable tools from //tvix:shell.

Change-Id: Ie042097a0d6fc26542faa96c139b77298ccb160a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11582
Reviewed-by: edef <edef@edef.eu>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/shell.nix

@@ -29,12 +29,10 @@ pkgs.mkShell {
     pkgs.cargo
     pkgs.cargo-machete
     pkgs.cargo-expand
-    pkgs.cbtemulator
     pkgs.clippy
     pkgs.evans
     pkgs.fuse
     pkgs.go
-    pkgs.google-cloud-bigtable-tool
     pkgs.grpcurl
     pkgs.hyperfine
     pkgs.mdbook