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>

Makefile

@@ -3,4 +3,4 @@ TARGETARCH ?= amd64
 LDFLAGS := -s -w -extldflags "-static"
 
 build:
-	CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -ldflags '${LDFLAGS}' -v -a -tags netgo -o docker-buildx ./cmd/docker-buildx
+	CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -ldflags '${LDFLAGS}' -v -a -tags netgo -o plugin-docker-buildx ./cmd/docker-buildx