Avoid update and upgrades to brew (#17815)

Ci is currently failing on brew update with the error:
```
Error: Cannot install bazelisk because conflicting formulae are installed.
  bazel: because Bazelisk replaces the bazel binary

Please `brew unlink bazel` before continuing.

Unlinking removes a formula's symlinks from /usr/local. You can
link the formula again after the install finishes. You can --force this
install, but the build may fail or cause obscure side effects in the
resulting software.
```
Avoiding:
```
$ brew update
$ brew upgrade
```
solves the issue by preventing the risk of conflicting formulae

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