Currently using envsubst to perform substitution of value overrides in
the feature gate caused conflicts as gotpl gets templated into those
overrides. This adds in '%%%REPLACE_${var}%%%' and uses sed to perform
the substitution instead to address the issue.
This is to achieve parity with OSH-infra patch in [0].
[0] https://review.opendev.org/#/c/697749/
Depends-On: https://review.opendev.org/#/c/697749
Change-Id: I3ed504c65900e7b84728019f3acdf706a40c0427
Signed-off-by: Tin Lam <tlam@omegaprime.dev>