Fix version reference in Dockerfile for E2E tests

[ycombinator]

In the Dockerfile used for E2E tests, there are two commands that copy the built Fleet Server artifact into the appropriate Agent components folder:

fleet-server/dev-tools/e2e/Dockerfile

Lines 6 to 7 in 8eb65da

	ONBUILD COPY --chmod=0755 --chown=elastic-agent cover/fleet-server-${STACK_VERSION}-linux-x86_64/fleet-server \
	./data/elastic-agent-${VCS_REF_SHORT}/components/fleet-server

fleet-server/dev-tools/e2e/Dockerfile

Lines 12 to 13 in 8eb65da

	ONBUILD COPY --chmod=0755 --chown=elastic-agent cover/fleet-server-${STACK_VERSION}-linux-arm64/fleet-server \
	./data/elastic-agent-${VCS_REF_SHORT}/components/fleet-server

The value of ${STACK_VERSION} mentioned in these commands should be the version of the built Fleet Server artifact, which should be the same as:

fleet-server/version/version.go

Line 9 in 8eb65da

const DefaultVersion = "9.0.0"

However, this isn't always the case, e.g. #3857 (comment)

We should fix this reference to contain the correct version value.

[ycombinator]

cc: @michel-laterman per our 1-1 discussion today.