Fix powershell syntax error

Author: moskyb

packer/windows/conf/bin/bk-install-elastic-stack.ps1

@@ -164,7 +164,7 @@ If (![string]::IsNullOrEmpty($Env:BUILDKITE_AGENT_SIGNING_KEY_PATH)) {
   Add-Content -Path C:\buildkite-agent\buildkite-agent.cfg -Value "signing-jwks-file=$keyfile"
 }
 
-if (![string]::IsNullOrEmpty)($Env:BUILDKITE_AGENT_SIGNING_KEY_ID) {
+if (![string]::IsNullOrEmpty($Env:BUILDKITE_AGENT_SIGNING_KEY_ID)) {
   Add-Content -Path C:\buildkite-agent\buildkite-agent.cfg -Value "signing-jwks-key-id=$Env:BUILDKITE_AGENT_SIGNING_KEY_ID"
 }
 

templates/aws-stack.yml

@@ -1201,9 +1201,9 @@ Resources:
                   $Env:BUILDKITE_AGENT_TIMESTAMP_LINES="${BuildkiteAgentTimestampLines}"
                   $Env:BUILDKITE_AGENT_EXPERIMENTS="${BuildkiteAgentExperiments}"
                   $Env:BUILDKITE_AGENT_TRACING_BACKEND="${BuildkiteAgentTracingBackend}"
-                  $Env:BUILDKITE_AGENT_SIGNING_KEY_PATH="${BuildkiteAgentSigningKeySSMParameter}" \
-                  $Env:BUILDKITE_AGENT_SIGNING_KEY_ID="${BuildkiteAgentSigningKeyID}" \
-                  $Env:BUILDKITE_AGENT_VERIFICATION_KEY_PATH="${BuildkiteAgentVerificationKeySSMParameter}" \
+                  $Env:BUILDKITE_AGENT_SIGNING_KEY_PATH="${BuildkiteAgentSigningKeySSMParameter}"
+                  $Env:BUILDKITE_AGENT_SIGNING_KEY_ID="${BuildkiteAgentSigningKeyID}"
+                  $Env:BUILDKITE_AGENT_VERIFICATION_KEY_PATH="${BuildkiteAgentVerificationKeySSMParameter}"
                   $Env:BUILDKITE_AGENT_RELEASE="${BuildkiteAgentRelease}"
                   $Env:BUILDKITE_QUEUE="${BuildkiteQueue}"
                   $Env:BUILDKITE_AGENT_ENABLE_GIT_MIRRORS="${BuildkiteAgentEnableGitMirrors}"