build-agents.sh: fix VERSION parsing with awk (was broken sed backslash)

Sed escape-sequence handling through the heredoc lost the \1
backreference, yielding an empty VERSION. Switched to
awk -F'"' '/^version/{print $2; exit}' which is simpler and resistant to
quoting. First full end-to-end signed build validated v0.6.1 deployed
and verified against the Microsoft cert chain.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

projects/msp-tools/guru-rmm/signing-attestation/signing-config/build-agents.sh

@@ -19,7 +19,7 @@ cd "$REPO_DIR"
 sudo -u guru git fetch origin 2>&1 | tee -a "$LOG_FILE"
 sudo -u guru git reset --hard origin/main 2>&1 | tee -a "$LOG_FILE"
 
-VERSION=$(grep '^version' agent/Cargo.toml | head -1 | sed 's/.*"\(.*\)".*/
/')
+VERSION=$(awk -F'"' '/^version/{print $2; exit}' agent/Cargo.toml)
 log "Building version: $VERSION"
 
 # Build Linux agent