fix(ci): set docker TARGET_ARCH

[FedeDP]

What type of PR is this?

/kind bug

Any specific area of the project related to this PR?

/area CI

What this PR does / why we need it:

In buildx TARGET_ARCH was automatically set.
Now that we use plain docker build, we must set it.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[FedeDP]

Solution tested in this run: https://github.com/falcosecurity/falco/actions/runs/5025201159/jobs/9011863784?pr=2557 and with the workflow:

build-minimal:
    runs-on: ubuntu-20.04
    env:
      TARGET_ARCH: ${{ (github.base_ref == 'master' && 'arm64') || 'amd64' }}
    steps:
      - name: test
        run: echo "${TARGET_ARCH}"

[FedeDP]

/milestone 0.35.0

[LucaGuerra]

LGTM

[Andreagit97]

/approve

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Andreagit97, FedeDP, jasondellaluce

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