fix(ci): wire AMDGPU_TARGETS through backend build workflow (#9445)

Commit 8839a71c exposed AMDGPU_TARGETS as an ARG/ENV in
Dockerfile.llama-cpp so GPU targets could be overridden, but never
wired the value through the CI workflow inputs. Without it, Docker
receives AMDGPU_TARGETS="" which overrides the Makefile's ?= default,
causing all hipblas builds to compile only for gfx906 regardless of
the target list in the Makefile.

Add amdgpu-targets as a workflow_call input with the same default list
as the Makefile, and pass it as AMDGPU_TARGETS in the build-args of
both the push and PR build steps.

Assisted-by: Claude Code:claude-sonnet-4-6

Signed-off-by: Russell Sim <rsl@simopolis.xyz>