Add script to install the latest version of jsctl

[inteon]

This is how we recommend the user installs jsctl currently:

$ VERSION="0.1.15"
$ ASSET_REF="jsctl-$VERSION-$(uname -s)-$(uname -m)"
$ ARCHIVE="$ASSET_REF.tar.gz"
$ curl -LO https://github.com/jetstack/jsctl/releases/download/v$VERSION/$ARCHIVE
$ tar -zxvf $ARCHIVE
$ sudo mv ./$ASSET_REF/jsctl /usr/local/bin/jsctl

This does not work for me since uname -s returns Linux instead of linux and that causes the sudo mv command to fail.

In #51 we were saying not to create a one-line install script, however I'm convinced that the advantages (simple & easy-to-use) outweigh the disadvantages (potentially insecure -> which is a moot point since the script is used to download and run our binary on the machine).

I renamed the release artefacts such that they don't contain the version anymore, this makes it easier to download the latest version directly. I also removed the architecture renaming from .goreleaser.yml, that is now done in the script instead. I also removed some target architectures that I think are not used (anymore) like windows 32 bit etc.

After creating a new release, the one liners should be:

curl -fsSL https://raw.githubusercontent.com/jetstack/jsctl/main/install.sh | sh

and

irm https://raw.githubusercontent.com/jetstack/jsctl/main/install.ps1 | iex

[irbekrm]

Thanks for working on this @inteon !

I agree with the general approach of having an install script and changing the file names make sense for me.
I think we want to keep Linux arm builds though, see my comment- let me know what you think

[irbekrm]

Note: in this case returning the latest release should be what we always want as there are no patch releases for non-latest minor/major releases.

[irbekrm]

Thanks for working on this @inteon !

I've played around with the script a bit, all seems to work as expected 👍🏼

Happy to review any follow-up fixes if needed.

/lgtm