feat: add linux/riscv64 to bin-image-cross release target

Add linux/riscv64 to the bin-image-cross platforms list so that
official release images include riscv64 binaries.

riscv64 is already present in the _platforms variable (line 38) and
used by the cross, dynbinary-cross, and plugins-cross targets. CI
already builds riscv64 binaries, but they are excluded from the
release image because bin-image-cross has its own platform list.

Closes #6857

Signed-off-by: Bruno Verachten <gounthar@gmail.com>

Author: gounthar

docker-bake.hcl

@@ -190,6 +190,7 @@ target "bin-image-cross" {
     "linux/arm/v7",
     "linux/arm64",
     "linux/ppc64le",
+    "linux/riscv64",
     "linux/s390x",
     "windows/amd64",
     "windows/arm64"