add spire_disabled build tag for entrypoint command

[jkhelil]

Add build tag to disable SPIRE (github.com/tektoncd/pipeline/pkg/spire) in cmd/entrypoint/main.go

Changes

This PR introduces a build tag to conditionally disable SPIRE functionality in the cmd/entrypoint/main.go file. The following changes have been made:

• Added a build tag disable_spire to conditionally exclude SPIRE-related logic.
• Split SPIRE logic into two files:
  spire.go for when SPIRE is enabled.
  spire_disabled.go for when SPIRE is disabled using the disable_spire build tag.

fixes #8531

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• Has Tests included if any functionality added or changed
• pre-commit Passed
• Follows the commit message standard
• Meets the Tekton contributor standards (including functionality, content, code)
• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings). See some examples of good release notes.
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[tekton-robot]

The following Tekton test failed:

Test name	Commit	Details	Required	Rerun command
check-pr-has-kind-label	294dcf8	link	true	/test check-pr-has-kind-label

[tekton-robot]

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[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage-df to re-run this coverage report

File	Old Coverage	New Coverage	Delta
cmd/entrypoint/main.go	11.9%	12.5%	0.6
cmd/entrypoint/spire.go	Do not exist	0.0%
pkg/entrypoint/entrypointer.go	88.1%	88.0%	-0.0
pkg/entrypoint/spire.go	Do not exist	100.0%