[go_sdk download] allow patches to standard library

[tyler-french]

What type of PR is this?

Feature

What does this PR do? Why is it needed?

Allows users to provide patches to the Go SDK when using the go_download_sdk rule or the go_sdk.download Bzlmod tag.

This is helpful when some modification is needed to a package in the go standard library, but the user does not want to manage a fork of the standard library itself.

Since we build the standard library from source, the patches should apply appropriately across multiple OS and Architectures. However, we should not expect patches to apply correctly across multiple versions of the Go toolchain. For that reason, we only allow patches if the version is specified.

Which issues(s) does this PR fix?

Fixes #3683

Testing
Added a test to the BCR which patches the os package and then runs a test to assert the patch properly applied.

Show that patches cannot apply when version is not specified:

% bazel-6.3.2 test //...
...
ERROR: An error occurred during the fetch of repository 'rules_go~override~go_sdk~rules_go_bcr_tests__download_0':
   Traceback (most recent call last):
        File "/home/user/.cache/bazel/_bazel_tfrench/565e6f63e863d6a7567b33b30501a80c/external/rules_go~override/go/private/sdk.bzl", line 93, column 21, in _go_download_sdk_impl
                fail("version must be specified when patches are specified")
Error in fail: version must be specified when patches are specified
...
ERROR: Couldn't start the build. Unable to run tests