Ensure that the plugin binary does not conflict with the utility being used (#24)

use methode described in https://docs.docker.com/build/buildx/install/ to install buildx into image.
- `/usr/lib/docker/cli-plugins/docker-buildx` -> `/usr/libexec/docker/cli-plugins/docker-buildx`
- fix linux/amd64 build on linux/arm64

rename plugin binary
- `docker-buildx` -> `plugin-docker-buildx`

Reviewed-on: https://codeberg.org/woodpecker-plugins/plugin-docker-buildx/pulls/24
Reviewed-by: anbraten <anbraten@noreply.codeberg.org>

.gitignore

@@ -1,6 +1,6 @@
 /release/
 
-docker-buildx
+plugin-docker-buildx
 
 coverage.out
 CHANGELOG.md