[20.10 backport] rpm: scan-cli-plugin: fix build-time "version" and "commit" variables

[thaJeztah]

backport of #549

When building rpm (and deb) packages, binaries are built from a source tarball.
Source tarballs will only include the raw source code, but do not include the
git repository;

docker-ce-packaging/rpm/Makefile

Lines 110 to 116 in c4c2d89

	rpmbuild/SOURCES/scan-cli-plugin.tgz:
	mkdir -p $(@D)
	docker run --rm -w /v \
	-v $(realpath $(CURDIR)/../src/github.com/docker/scan-cli-plugin):/scan-cli-plugin \
	-v $(CURDIR)/$(@D):/v \
	alpine \
	tar -C / -c -z -f /v/scan-cli-plugin.tgz --exclude .git scan-cli-plugin

tar -C / -c -z -f /v/scan-cli-plugin.tgz --exclude .git scan-cli-plugin

Because of this, the makefile used to build the scan-cli-plugin binaries will
not be able to determine the "commit" (and version) to set as build-time variable
to include in the --version output; https://github.com/docker/scan-cli-plugin/blob/3eaac3e88412543fb18767feb697e11dca21d20f/builder.Makefile#L5-L19
resulting in both version and git commit to be empty in the version output:

docker scan --version
Version:
Git commit:
Provider:   Snyk (1.563.0 (standalone))

This patch updates the RPM spec to add variables for passing this information,
and collects the commit and version from the host to pass it to the build-
container in which the RPMs are built, similar to how they're passed for the
equivalent "deb" build-containers (which use env-vars for this);

docker-ce-packaging/deb/Makefile

Lines 36 to 37 in c4c2d89

	-e SCAN_VERSION=$(DOCKER_SCAN_REF) \
	-e SCAN_GITCOMMIT=$(SCAN_GITCOMMIT) \

Before:

docker scan --version
Version:
Git commit:
Provider:   Snyk (1.563.0 (standalone))

docker info
Client:
 Context:    default
 Debug Mode: false
 Plugins:
  app: Docker App (Docker Inc., v0.9.1-beta3)
  buildx: Build with BuildKit (Docker Inc., v0.5.1-docker)
  scan: Docker Scan (Docker Inc.)
...

After:

docker scan --version
Version:    v0.8.0
Git commit: 35651ca
Provider:   Snyk (1.563.0 (standalone))

docker info

Client:
 Context:    default
 Debug Mode: false
 Plugins:
  app: Docker App (Docker Inc., v0.9.1-beta3)
  buildx: Build with BuildKit (Docker Inc., v0.5.1-docker)
  scan: Docker Scan (Docker Inc., v0.8.0)
...

(cherry picked from commit 20c5fee)

[glours]

LGTM

[fredericdalleau]

LGTM