Refer to default branch in links with HEAD

Anywhere we can use `blob/master` we can use `blob/HEAD` instead. This
will make life easier if we ever rename our default branch in future
(once/if Git and GitHub provides the necessary tooling to do so).

Sorbet: update hidden definitions

test/support/fixtures/bottles: add Big Sur fixtures

linkage_checker: handle system libraries on Big Sur

os/mac/utils/bottles: support ARM bottles.

This will be eventually needed for Big Sur.

shims/scm: handle missing Subversion on Catalina.

Use the shim to find a valid Subversion.

Fixes https://github.com/Homebrew/brew/issues/7781

Integrate `--cache` and `home` with cask

Use Utils.safe_popen_read instead of Utils.popen_read

OS::Mac::Version: Change 11.00 to 11.0

os/mac/pkgconfig: add pc files for Big Sur

os/mac/pkgconfig/10.15/expat: update version to 2.2.8

build(deps): bump diff-lcs from 1.4 to 1.4.2 in /Library/Homebrew

Bumps [diff-lcs](https://github.com/halostatue/diff-lcs) from 1.4 to 1.4.2.
- [Release notes](https://github.com/halostatue/diff-lcs/releases)
- [Changelog](https://github.com/halostatue/diff-lcs/blob/master/History.md)
- [Commits](https://github.com/halostatue/diff-lcs/compare/v1.4...v1.4.2

)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

RuboCop requires using Utils.safe_popen_read and Utils.safe_popen_write
instead of Utils.popen_read and Utils.popen_write respectively.

Using the "safe" version means that an error will be shown if the
command fails. Previously, when using `popen_read`, a failed command can
go unnoticed and have negative consequences that go undetected (as
happened for pipend in
https://github.com/Homebrew/homebrew-core/pull/55682)

RuboCop does not require Utils.safe_popen_read in a test do block
because there can be some legitimate uses for Utils.popen_read in these
cases.

os/mac: fix for major versions other than 10

formula: set GOPATH

So we can cache dependencies and speed up subsequent builds.

os/mac/xcode: add Big Sur clang versions.

https://github.com/Homebrew/brew/pull/7806#issuecomment-648154860

macOS Big Sur tweaks