Directly invoke make during the container build

I don't know how the upstream build ever worked.

This change is intended to get GitHub Actions building, as the first
attempt failed because it couldn't find .build/linux-amd64. That
directory doesn't get created during a local build and doesn't appear to
be mentioned in any capacity whatsoever in this repository.

Dockerfile

@@ -5,7 +5,8 @@ LABEL maintainer="The Prometheus Authors <[email protected]
 
 ARG ARCH="amd64"
 ARG OS="linux"
-COPY .build/${OS}-${ARCH}/mysqld_exporter /bin/mysqld_exporter
+RUN make
+COPY mysqld_exporter /bin/mysqld_exporter
 
 EXPOSE      9104
 USER        nobody