build: Switch to build args over env vars in dockerfile — heartwood

--env is a podman-specific feature. While it is clear that podman is a requirement here, switching these to build arguments better expresses intent.

These values are meant to be configurable at build time, which is exactly what build arguments are for, while environment variables are meant to persist in the image built.

Additionally, this removes a couple of unused env vars.

 WORKDIR /src
 COPY . .
 ARG TZ
 ARG LC_ALL
 ARG SOURCE_DATE_EPOCH
 ARG RADICLE_VERSION
 # Copy cargo configuration we're going to use to specify compiler options.
 RUN mkdir -p .cargo && cp build/config.toml .cargo/config.toml
 # Install dependencies.

 # Now copy the files to a new image without all the intermediary artifacts to
 # save some space.
 FROM alpine:3.19 as packager
 ARG RADICLE_VERSION
 ARG SOURCE_DATE_EPOCH
 COPY --from=builder \
      /src/target/x86_64-unknown-linux-musl/release/rad \
      /src/target/x86_64-unknown-linux-musl/release/git-remote-rad \

     find * -maxdepth 0 -type d -exec tar \
     --sort=name \
     --verbose \
     --mtime="@$GIT_COMMIT_TIME" \
     --mtime="@$SOURCE_DATE_EPOCH" \
     --owner=0 \
     --group=0 \
     --numeric-owner \

   echo "Building image ($image).."
   podman --cgroup-manager=cgroupfs build \
     --env SOURCE_DATE_EPOCH \
     --env TZ \
     --env LC_ALL \
     --env GIT_COMMIT_TIME=$SOURCE_DATE_EPOCH \
     --env GIT_HEAD=$rev \
     --env RADICLE_VERSION=$version \
     --build-arg SOURCE_DATE_EPOCH \
     --build-arg TZ \
     --build-arg LC_ALL \
     --build-arg RADICLE_VERSION=$version \
     --arch amd64 --tag $image -f ./build/Dockerfile - < $gitarchive
   echo "Creating container (radicle-build-container).."