Switch to larger Helios instances for CI (#10018)

This PR switches the slowest Helios jobs to run on larger instance
sizes, significantly speeding up CI times. Along with this I **already
deployed** a Buildomat configuration change to run all Helios jobs on
Zen 4 AWS instances, instead of Zen 3 instances either on AWS or lab
Gimlets. Together, these two changes should bring CI times down
considerably.

Unfortunately we cannot use Zen 5 AWS instances (like we did on Linux)
until oxidecomputer/stlouis#938 is fixed.

Author: emilyalbini

.github/buildomat/jobs/build-and-test-helios.sh

@@ -2,7 +2,7 @@
 #:
 #: name = "build-and-test (helios)"
 #: variety = "basic"
-#: target = "helios-2.0"
+#: target = "helios-2.0-32c256gb"
 #: rust_toolchain = true
 #: output_rules = [
 #:	"%/work/*",

.github/buildomat/jobs/package.sh

@@ -2,7 +2,7 @@
 #:
 #: name = "helios / package"
 #: variety = "basic"
-#: target = "helios-2.0"
+#: target = "helios-2.0-16c64gb"
 #: rust_toolchain = true
 #: output_rules = [
 #:	"=/work/package.tar.gz",

.github/buildomat/jobs/tuf-repo.sh

@@ -2,7 +2,7 @@
 #:
 #: name = "helios / build TUF repo"
 #: variety = "basic"
-#: target = "helios-2.0"
+#: target = "helios-2.0-16c128gb"
 #: rust_toolchain = true
 #: output_rules = [
 #:	"=/work/manifest.toml",