chore: build using only amd64 builders

Our CI amd64 builders are 3-4 times faster to build Talos than our arm64
builders when building Talos.

Our Dockerfile was restructured a while ago to support cross-compilation
on all platforms but CI was still using amd64/arm64 workers, so arm64
part was done on arm64 builders.

As our CI runs on Talos, `binfmt_misc` is not enabled in the kernel, but
buildkit has built-in QEMU emulation layer which works just fine for
those small pieces which actually need to run arm64 binaries on amd64
(mostly `apk add` in the installer container). Interesting enough,
buildkit QEMU support fails for `ca-certificates` script which runs
after install. At the same time I believe we don't need
`ca-certificates` in the installer, as installer doesn't download
anything from the network, and `ca-certificates` were added a while ago
when installer was actually downloading configuration on its own.

Signed-off-by: Andrey Smirnov <andrey.smirnov@talos-systems.com>

.drone.jsonnet

@@ -136,6 +136,9 @@ local setup_ci = {
     'make ./_out/sonobuoy',
     'make ./_out/kubectl',
   ],
+  environment: {
+    "BUILDKIT_FLAVOR": "cross",
+  },
   volumes: volumes.ForStep(),
 };
 

Dockerfile

@@ -509,9 +509,8 @@ COPY --from=pkg-grub / /
 COPY --from=unicode-pf2 /usr/share/grub/unicode.pf2 /usr/share/grub/unicode.pf2
 
 FROM alpine:3.14.1 AS installer
-RUN apk add --no-cache --update \
+RUN apk add --no-cache --update --no-scripts \
     bash \
-    ca-certificates \
     efibootmgr \
     mtools \
     qemu-img \