Provide mechanism to override --dist-tag in lerna publish

[sambostock]

shipit-engine/lib/snippets/publish-lerna-independent-packages

Line 33 in f8f47fc

"--dist-tag latest",

hard codes the latest tag.

This is problematic for stacks publishing backports to previous versions of a package.

There should be a way to inject a custom value to be passed to --dist-tag.

Something similar exists in

shipit-engine/app/models/shipit/deploy_spec/npm_discovery.rb

Lines 56 to 63 in f8f47fc

	def dist_tag(version)
	# Pre-release SemVer tags such as 'beta', 'alpha', 'rc' and 'next'
	# are treated as 'next' npm dist-tags.
	# An 1.0.0-beta.1 would be installable using both:
	# `yarn add package@1.0.0-beta.1` and `yarn add package@next`
	return 'next' if ['-beta', '-alpha', '-rc', '-next'].any? { |tag| version.include?(tag) }
	'latest'
	end

but this too would be insufficient, as a backport stack would need to define an entirely custom tag (e.g. v5-stable or 5.x).

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Relevant to https://github.com/Shopify/sewing-kit/pull/2954
cc. @BPScott