build(builder): pin base image version to debian:buster

[leogr]

What type of PR is this?

/kind bug

Any specific area of the project related to this PR?

/area build

What this PR does / why we need it:

Currently, the publishing of the builder image is broken (see the failing test on the master branch).

So, this PR introduces a simple workaround to make the builder image work again.
The root issue is due to debian:stable which recently moved forward and removed clang7. So we just pin the last working version, which is debian:buster. We already did the same thing on Falco, see falcosecurity/falco#1719.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Upgrading to a newer clang version is out-of-scope for this PR. The intent is just to make the tool work again shortly.

Does this PR introduce a user-facing change?:

NONE

[poiana]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[leogr]

Sorry, I did not realize this is a dup of #109 :/

So,
/close

[poiana]

@leogr: Closed this PR.

In response to this:

Sorry, I did not realize this is a dup of #109 :/

So,
/close

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