fix(scripts): modify logic for determining terraform arch (#9595)

Commit fb3616c used a build-arg to set the desired terrafomr
architeture to fetch. This is set in build_docker.sh but not
when building using depot.dev. Baking the logic into the
Dockerfile instead.

scripts/build_docker.sh

@@ -103,7 +103,6 @@ declare -A arch_map=(
 	[arm]="linux/arm/v7"
 	[armv7]="linux/arm/v7"
 )
-orig_arch="$arch"
 if [[ "${arch_map[$arch]+exists}" != "" ]]; then
 	arch="${arch_map[$arch]}"
 fi
@@ -125,7 +124,6 @@ if [[ "$build_base" != "" ]]; then
 	log "--- Building base Docker image for $arch ($build_base)"
 	docker build \
 		--platform "$arch" \
-		--build-arg "ARCH=${orig_arch}" \
 		--tag "$build_base" \
 		--no-cache \
 		-f Dockerfile.base \