Use htmltest binary not build from source

[markmandel]

What type of PR is this?

Uncomment only one /kind <> line, press enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking
/kind bug

/kind cleanup

/kind documentation
/kind feature
/kind hotfix

What this PR does / Why we need it:

We just got a new release of htmltest, so we no longer need to build the binary from source. Shrinks the time to build the build image by quite a bit, as we don't have as much work to do.

Which issue(s) this PR fixes:

N/A

Special notes for your reviewer:

N/A

[quilkin-bot]

Build Succeeded 🥳

Build Id: 0eab45d2-4a6a-4cb1-9fb3-a979e08855a1

To build this version:

git fetch git@github.com:googleforgames/quilkin.git pull/516/head:pr_516 && git checkout pr_516
cargo build