Fix plugin version showing unspecified

[rishabh998186]

Downloaded plugin binaries currently show unspecified version information:

$ curl -Lo wait_plugin https://github.com/pipe-cd/pipecd/releases/download/pkg%2Fapp%2Fpipedv1%2Fplugin%2Fwait%2Fv0.1.1/wait_v0.1.1_darwin_arm64
$ ./wait_plugin version
Version: unspecified, GitCommit: unspecified, BuildDate: unspecified

This makes it impossible to verify which version of a plugin is deployed.

Root Cause
The plugin_release workflow wasn't passing version information when building plugins, and the Makefile's build/plugin target didn't inject version details via ldflags like the build/go target does.

Changes
1 - Updated Makefile
Modified the build/plugin target to inject version/commit/buildDate via ldflags, using the same pattern as build/go:

• Added BUILD_VERSION, BUILD_COMMIT, BUILD_DATE variables

• Updated BUILD_OPTS to include version ldflags

2 - Updated workflow
Modified .github/workflows/plugin_release.yaml to pass BUILD_VERSION=${{ inputs.version }} to all plugin build commands (all 4 platform targets)

Test
Verified the fix works locally:

$ make build/plugin PLUGINS=wait BUILD_VERSION=v0.1.1-test
$ ./.artifacts/plugins/wait version
Version: v0.1.1-test, GitCommit: 03df4a5a6d7be22a238c88fe3d741bbf3fb27e20, BuildDate: 20251010-074520

Also tested with the kubernetes plugin to confirm the fix works generically across all plugins:

$ make build/plugin PLUGINS=kubernetes BUILD_VERSION=v0.2.0-test
$ ./.artifacts/plugins/kubernetes version
Version: v0.2.0-test, GitCommit: 03df4a5a6d7be22a238c88fe3d741bbf3fb27e20, BuildDate: 20251010-074748
 

fixes #6306

[khanhtc1202]

LGTM, thank you 👍