Add set -e to Frontier build scripts for fail-fast behavior

Without set -e, the benchmark build loop could silently ignore failures
of earlier benchmarks if a later one succeeded, since only the last
command's exit code would propagate.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

.github/workflows/frontier/build.sh

@@ -1,5 +1,7 @@
 #!/bin/bash
 
+set -e
+
 # Ignore SIGHUP to survive login node session drops
 trap '' HUP
 

.github/workflows/frontier_amd/build.sh

@@ -1,5 +1,7 @@
 #!/bin/bash
 
+set -e
+
 # Ignore SIGHUP to survive login node session drops
 trap '' HUP