test(secrets): serialize env-var-mutating test classes into one xunit collection

Environment.SetEnvironmentVariable/GetEnvironmentVariable mutate the
process-wide environ; concurrent setenv/getenv across parallel xunit test
classes is a documented crash/flake source on glibc even with unique
variable names. Group the six classes that touch process env vars into a
DisableParallelization=true collection so they run serially against each
other while everything else keeps running in parallel.

ZB.MOM.WW.Secrets/tests/ZB.MOM.WW.Secrets.Tests/DependencyInjection/AddZbSecretsTests.cs

@@ -12,6 +12,7 @@ namespace ZB.MOM.WW.Secrets.Tests.DependencyInjection;
 /// core: every seam resolves, a real secret round-trips through the composed graph, and a missing
 /// master key fails closed.
 /// </summary>
+[Collection("Process environment")]
 public sealed class AddZbSecretsTests
 {
     private static string NewTempDbPath() =>