ci(osx): use new location of the `perforce` cask

The Azure Pipelines builds are failing for macOS due to a change in the
location of the perforce cask. The command outputs the following error:

    + brew install caskroom/cask/perforce
    Error: caskroom/cask was moved. Tap homebrew/cask-cask instead.

So let's try to call `brew cask install perforce` first (which is what
that error message suggests, in a most round-about way).

Prior to 672f51cb we used to install the 'perforce' package with 'brew
install perforce' (note: no 'cask' in there). The justification for
672f51cb was that the command 'brew install perforce' simply stopped
working, after Homebrew folks decided that it's better to move the
'perforce' package to a "cask". Their justification for this move was
that 'brew install perforce' "can fail due to a checksum mismatch ...",
and casks can be installed without checksum verification. And indeed,
both 'brew cask install perforce' and 'brew install
caskroom/cask/perforce' printed something along the lines of:

  ==> No checksum defined for Cask perforce, skipping verification

It is unclear why 672f51cb used 'brew install caskroom/cask/perforce'
instead of 'brew cask install perforce'. It appears (by running both
commands on old Travis CI macOS images) that both commands worked all
the same already back then.

In any case, as the error message at the top of this commit message
shows, 'brew install caskroom/cask/perforce' has stopped working
recently, but 'brew cask install perforce' still does, so let's use
that.

CI servers are typically fresh virtual machines, but not always. To
accommodate for that, let's try harder if `brew cask install perforce`
fails, by specifically pulling the latest `master` of the
`homebrew-cask` repository.

This will still fail, of course, when `homebrew-cask` falls behind
Perforce's release schedule. But once it is updated, we can now simply
re-run the failed jobs and they will pick up that update.

As for updating `homebrew-cask`: the beginnings of automating this in
https://dev.azure.com/gitgitgadget/git/_build?definitionId=11&_a=summary
will be finished once the next Perforce upgrade comes around.

Helped-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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