Local: lookup for bindle-server uses amd64 build if os=linux

[vdice]

For the local installer scenario, the lookup method for fetching the bindle-server binary grabs the checked-in linux/amd64 build when the os is linux:

installer/local/job/bindle.nomad

Lines 61 to 63 in 0482aa0

	source = lookup({
	linux="https://raw.githubusercontent.com/fermyon/installer/93008bc6461076da5c5f7f99cffc3e68b1955a17/local/bindle/bindle-server"
	}, var.os, "https://bindle.blob.core.windows.net/releases/bindle-v0.8.0-${var.os}-${var.arch}.tar.gz")

This causes errors for linux/aarch64 environments.

Update the lookup to also inspect arch.

Note that linux/aarch64 binaries are only available as of bindle v0.8.2, so we would have to bump the version in tandem.