ci: don't update Homebrew

Lately our GCC macOS build job on Travis CI has been erroring out
while installing dependencies with:

  +brew link gcc@8
  Error: No such keg: /usr/local/Cellar/gcc@8
  The command "ci/install-dependencies.sh" failed and exited with 1 during .

Now, while gcc@8 is still pre-installed (but not linked) and would be
perfectly usable in the Travis CI macOS image we use [1], it's at
version 8.2.  However, when installing dependencies we first
explicitly run 'brew update', which spends over two minutes to update
itself and information about the available packages, and it learns
about GCC 8.3.  After that point gcc@8 exclusively refers to v8.3,
and, unfortunately, 'brew' is just too dumb to be able to do anything
with the still installed 8.2 package, and the subsequent 'brew link
gcc@8' fails.  (Even 'brew uninstall gcc@8' fails with the same
error!)

Don't run 'brew update' to keep the already installed GCC 8.2 'brew
link'-able.  Note that in addition we have to 'export
HOMEBREW_NO_AUTO_UPDATE=1' first, because 'brew' is so very helpful
that it would implicitly run update for us on the next 'brew install
<pkg>' otherwise.

Disabling 'brew update' has additional benefits:

  - It shaves off 2-3mins from the ~4mins currently spent on
    installing dependencies, and the macOS build jobs have always been
    prone to exceeding the time limit on Travis CI.

  - Our builds won't suddenly break because of the occasional Homebrew
    breakages [2].

The drawback is that we'll be stuck with slightly older versions of
the packages that we install via Homebrew (Git-LFS 2.5.2 and Perforce
2018.1; they are currently at 2.7.2 and 2019.1, respectively).  We
might want to reconsider this decision as time goes on and/or switch
to a more recent macOS image as they become available.

[1] 2000ac9fbf (travis-ci: switch to Xcode 10.1 macOS image,
    2019-01-17)

[2] See e.g. a1ccaedd62 (travis-ci: make the OSX build jobs' 'brew
    update' more quiet, 2019-02-02) or

    https://public-inbox.org/git/20180907032002.23366-1-szeder.dev@gmail.com/T/#+u

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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