ci: update brew before installing packages

[fanquake]

Given an older base image is being used, this can avoid instances where package installs fail, due to brew trying to use no-longer existent download locations.

Will see how long the update step takes, although a few minutes extra time spent in the macOS CI is probably less of an issue here. If it seems too long, I think moving to the Big Sur image would also be fine.

[fanquake]

Looks like the brew update step adds 4-5 minutes to the runtime of the macOS CIs, not terrible given they are run delayed / adhoc by Cirrus anyways. If that's acceptable then this PR could be mergable. The failure is in a Linux build that is not touched here.

One other thing I noticed, is that even though we're installinggcc@9, I'm pretty sure that Apple Clang (which is gcc on macOS) is being used for the builds.

[sipa]

@fanquake Pretty sure it's using GCC, see https://cirrus-ci.com/task/5799658971725824:

+ gcc-9 -v
Using built-in specs.
COLLECT_GCC=gcc-9
COLLECT_LTO_WRAPPER=/usr/local/Cellar/gcc@9/9.4.0/libexec/gcc/x86_64-apple-darwin19/9.4.0/lto-wrapper
Target: x86_64-apple-darwin19
Configured with: ../configure --prefix=/usr/local/Cellar/gcc@9/9.4.0 --libdir=/usr/local/Cellar/gcc@9/9.4.0/lib/gcc/9 --disable-nls --enable-checking=release --enable-languages=c,c++,objc,obj-c++,fortran --program-suffix=-9 --with-gmp=/usr/local/opt/gmp --with-mpfr=/usr/local/opt/mpfr --with-mpc=/usr/local/opt/libmpc --with-isl=/usr/local/opt/isl --with-pkgversion='Homebrew GCC 9.4.0' --with-bugurl=https://github.com/Homebrew/homebrew-core/issues --build=x86_64-apple-darwin19 --with-system-zlib --disable-multilib --with-native-system-header-dir=/usr/include --with-sysroot=/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk
Thread model: posix
gcc version 9.4.0 (Homebrew GCC 9.4.0) 

[sipa]

utACK 16af60a