[ci] Fix scheduled JDK matrix CI jobs (#15623)

This commit fixes failed scheduled JDK matrix CI jobs, that
can't access the default values for the OS and JDK from the input
steps, as observed in [^1] and [^2].

[^1] https://buildkite.com/elastic/logstash-linux-jdk-matrix-pipeline/builds/53#018c1371-b760-4c28-9203-340c0a1df150
[^2]: https://buildkite.com/elastic/logstash-windows-jdk-matrix-pipeline/builds/35#018c1371-b72e-48b4-b707-ce103eb6039c

(cherry picked from commit f0019bf)

.buildkite/linux_jdk_matrix_pipeline.yml

@@ -1,5 +1,9 @@
 # yaml-language-server: $schema=https://raw.githubusercontent.com/buildkite/pipeline-schema/main/schema.json
 
+env:
+  DEFAULT_MATRIX_OS: "ubuntu-2204"
+  DEFAULT_MATRIX_JDK: "adoptiumjdk_17"
+
 steps:
   - input: "Test Parameters"
     if: build.source != "schedule"
@@ -9,7 +13,7 @@ steps:
         hint: "The operating system variant(s) to run on:"
         required: true
         multiple: true
-        default: "ubuntu-2204"
+        default: "${DEFAULT_MATRIX_OS}"
         options:
           - label: "Ubuntu 22.04"
             value: "ubuntu-2204"
@@ -43,7 +47,7 @@ steps:
         hint: "The JDK to test with:"
         required: true
         multiple: true
-        default: "adoptiumjdk_17"
+        default: "${DEFAULT_MATRIX_JDK}"
         options:
           - label: "Adoptium JDK 17 (Eclipse Temurin)"
             value: "adoptiumjdk_17"
@@ -68,8 +72,8 @@ steps:
       python3 -m pip install ruamel.yaml
 
       echo "--- Printing generated dynamic steps"
-      export MATRIX_OSES="$(buildkite-agent meta-data get matrix-os)"
-      export MATRIX_JDKS="$(buildkite-agent meta-data get matrix-jdk)"
+      export MATRIX_OSES="$(buildkite-agent meta-data get matrix-os --default=${DEFAULT_MATRIX_OS})"
+      export MATRIX_JDKS="$(buildkite-agent meta-data get matrix-jdk --default=${DEFAULT_MATRIX_JDK})"
       set +eo pipefail
       python3 .buildkite/scripts/jdk-matrix-tests/generate-steps.py >pipeline_steps.yml
       if [[ $$? -ne 0 ]]; then

.buildkite/windows_jdk_matrix_pipeline.yml

@@ -1,5 +1,9 @@
 # yaml-language-server: $schema=https://raw.githubusercontent.com/buildkite/pipeline-schema/main/schema.json
 
+env:
+  DEFAULT_MATRIX_OS: "windows-2022"
+  DEFAULT_MATRIX_JDK: "adoptiumjdk_17"
+
 steps:
   - input: "Test Parameters"
     if: build.source != "schedule"
@@ -9,7 +13,7 @@ steps:
         hint: "The operating system variant(s) to run on:"
         required: true
         multiple: true
-        default: "windows-2022"
+        default: "${DEFAULT_MATRIX_OS}"
         options:
           - label: "Windows 2022"
             value: "windows-2022"
@@ -23,7 +27,7 @@ steps:
         hint: "The JDK to test with:"
         required: true
         multiple: true
-        default: "adoptiumjdk_17"
+        default: "${DEFAULT_MATRIX_JDK}"
         options:
           - label: "Adoptium JDK 17 (Eclipse Temurin)"
             value: "adoptiumjdk_17"
@@ -48,8 +52,8 @@ steps:
       python3 -m pip install ruamel.yaml
 
       echo "--- Printing generated dynamic steps"
-      export MATRIX_OSES="$(buildkite-agent meta-data get matrix-os)"
-      export MATRIX_JDKS="$(buildkite-agent meta-data get matrix-jdk)"
+      export MATRIX_OSES="$(buildkite-agent meta-data get matrix-os --default=${DEFAULT_MATRIX_OS})"
+      export MATRIX_JDKS="$(buildkite-agent meta-data get matrix-jdk --default=${DEFAULT_MATRIX_JDK})"
       set +eo pipefail
       python3 .buildkite/scripts/jdk-matrix-tests/generate-steps.py >pipeline_steps.yml
       if [[ $$? -ne 0 ]]; then