Support optional override of the target archictures

This can help speed up incremental builds when you're only testing one
archicture, like amd64.  E.g.
BUILD_ARCH=amd64 ./scripts/build_linux.sh && scp ./dist/ollama-linux-amd64 test-system:

scripts/build_linux.sh

@@ -5,9 +5,10 @@ set -eu
 export VERSION=${VERSION:-0.0.0}
 export GOFLAGS="'-ldflags=-w -s \"-X=github.com/jmorganca/ollama/version.Version=$VERSION\" \"-X=github.com/jmorganca/ollama/server.mode=release\"'"
 
+BUILD_ARCH=${BUILD_ARCH:-"amd64 arm64"}
 mkdir -p dist
 
-for TARGETARCH in amd64 arm64; do
+for TARGETARCH in ${BUILD_ARCH}; do
     docker build --platform=linux/$TARGETARCH --build-arg=GOFLAGS --build-arg=CGO_CFLAGS -f Dockerfile.build -t builder:$TARGETARCH .
     docker create --platform linux/$TARGETARCH --name builder-$TARGETARCH builder:$TARGETARCH
     docker cp builder-$TARGETARCH:/go/src/github.com/jmorganca/ollama/ollama ./dist/ollama-linux-$TARGETARCH