env/openbsd-amd64: support 386 build

Change-Id: I551acb269bc19600d71c2a9d5d5043e3d9ff973c
Reviewed-on: https://go-review.googlesource.com/c/build/+/528515
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Heschi Kreinick <heschi@google.com>
diff --git a/env/openbsd-amd64/make.bash b/env/openbsd-amd64/make.bash
index a352b4c..1243544 100755
--- a/env/openbsd-amd64/make.bash
+++ b/env/openbsd-amd64/make.bash
@@ -119,7 +119,7 @@
 EOF
 chmod +x ${SITE}/install.site
 mkdir -p ${SITE}/usr/local/bin
-CGO_ENABLED=0 GOOS=openbsd GOARCH=amd64 go1.21.0 build -o ${SITE}/usr/local/bin/bootstrapswarm golang.org/x/build/cmd/bootstrapswarm
+CGO_ENABLED=0 GOOS=openbsd GOARCH=${ARCH/i386/386} go1.21.0 build -o ${SITE}/usr/local/bin/bootstrapswarm golang.org/x/build/cmd/bootstrapswarm
 tar -C ${SITE} -zcf ${WORK}/site${RELNO}.tgz .
 
 # Autoinstall script.