Update JSON template does not return substitute values in 1.8.6 version

[nywilken]

This change fixes a regression introduced in Packer 1.8.6, where configurations custom builder names
, via the name attribute, would internally interpolate the proper build names, but would display the
name to STDOUT as uninterpolated values. The change updates the creation of a CoreBuild for legacy JSON templates
to use use the interpolated name as the final build type, which gets rendered to the screen.

Test Results Before Fix

~>  go test ./... -run='TestCoreBuild_buildNameIntepolation' -v
--- FAIL: TestCoreBuild_buildTypeIntepolation (0.01s)
    core_test.go:930: build type interpolation failed; expected "test.mybuild-RandomToken", got "test.{{user `build_name`}}"
    core_test.go:930: build type interpolation failed; expected "test.build-vardata", got "test.{{user `build_name`}}"
    core_test.go:930: build type interpolation failed; expected "test.build-12345", got "test.{{user `build_name`}}"
FAIL

Test Results After Fix

~>  go test ./... -run='TestCoreBuild_buildNameIntepolation' -v
=== RUN   TestCoreBuild_buildTypeIntepolation
--- PASS: TestCoreBuild_buildTypeIntepolation (0.01s)
PASS

Closes #12281

[lbajolet-hashicorp]

Good catch, LGTM!

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