ci: pin system-tests version [3.7] (#13436)

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Author: brettlangdon

.github/workflows/system-tests.yml

@@ -20,6 +20,7 @@ jobs:
         with:
           persist-credentials: false
           repository: 'DataDog/system-tests'
+          ref: '605be581d163b86ebe855143c7615b1718b7ab6e'
 
       - name: Build agent
         run: ./build.sh -i agent
@@ -66,6 +67,7 @@ jobs:
         with:
           persist-credentials: false
           repository: 'DataDog/system-tests'
+          ref: '605be581d163b86ebe855143c7615b1718b7ab6e'
 
       - name: Checkout dd-trace-py
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
@@ -118,6 +120,7 @@ jobs:
         with:
           persist-credentials: false
           repository: 'DataDog/system-tests'
+          ref: '605be581d163b86ebe855143c7615b1718b7ab6e'
 
       - name: Build runner
         uses: ./.github/actions/install_runner
@@ -299,6 +302,7 @@ jobs:
         with:
           persist-credentials: false
           repository: 'DataDog/system-tests'
+          ref: '605be581d163b86ebe855143c7615b1718b7ab6e'
       - name: Checkout dd-trace-py
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with: