Make get operation of env-store to be cancelable and single-flight

[nghialv]

What this PR does / why we need it:

• Add ctx to Get function to allow canceling the request
• Ensure that only one API call is executed for a given environment at a time

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[pipecd-bot]

Code coverage for golang is 32.58%. This pull request decreases coverage by -0.01%.

File	Function	Base	Head	Diff
pkg/app/piped/apistore/environmentstore/store.go	Store.Get	0.00%	0.00%	+0.00%

[nakabonne]

Nice. Your effort helps us.
/approve

[pipecd-bot]

This pull request is APPROVED by nakabonne.

Approvers can cancel the approval by writing /approve cancel in a comment. Any additional commits also will change this pull request to be not-approved.