adds deferred variable cache to common.mk

[jaxesn]

Issue #, if available:

Description of changes:

We use a decent amount of shell calls in out makefiles. Because we currently defer these, they get resolved a number of times, every time they are used in the context of the makefile. We could change these to immediately resolved variables, however that means we would exec these shell calls at the start of every make invocation, even tho they may not be needed for the specific target being called. This PR introduces a caching mechanism for these deferred variables and a helper to overwrite variables by name to be automatically cached on their first resolution.

Commits:

• adds helper and caches shell, or otherwise, expensive vars
• changes all project makefiles to defer git_tag and golang version since these are now cached after first use

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[abhay-krishna]

/lgtm
/approve

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