Allow overriding os_family in rbe_autoconfig

[kevingessner]

(Rebased and making a PR on behalf of @ulfjack, who wrote this patch)

Provides a workaround for #895.

Use like this:
rbe_autoconfig(
name = "engflow_remote_config",
os_family = "Linux",
java_home = "/usr/lib/jvm/11.29.3-ca-jdk11.0.2/reduced",
use_checked_in_confs = "Force",
)

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[smukherj1]

@kevingessner FYI we're exploring the possibility of exploring rbe_autoconfig for config generation with a standalone Go CLI (see PR #936 for a proof of concept). We expect this CLI will make it easier to generate configs (mostly for our internal config generation automation as well as for new users setting up config generation for themselves).

The CLI currently supports Linux & Windows to start out but I imagine Mac OS support should be trivial to add. We're still experimenting with this tool and will share more details in the coming weeks but initial thoughts are welcome.

Right now the Go code also potentially be used as a library and we would be open to suggestions on what components of the tool should be exported to make it more pluggable. This will be especially important for potential Windows/OSX specific support that we might not initially be able to set up testing for.