Remove Buildarch: read the architecture at run time (closes #83)

The architecture is no longer passed in at build time. The Buildarch
variable and field are gone from main and globals, script/build no
longer stamps it in with -X, and the startup and listen log lines
report runtime.GOARCH under the key "arch". The Dockerfile comment and
backend/README.md no longer describe an architecture being stamped in.

Model: opus-5-5

Dockerfile

@@ -37,7 +37,7 @@ RUN make test
 
 # make build is a shim around backend/script/build, the one definition
 # of the build command:
-# CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X main.Version=... -X main.Buildarch=..."
+# CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X main.Version=..."
 # That script reads VERSION from the environment, so it is handed over
 # there rather than as a make variable.
 ARG VERSION=dev