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/Cli/Interactive/LiteralMasterKeyProviderTests.cs

@@ -11,6 +11,7 @@ namespace ZB.MOM.WW.Secrets.Tests.Cli.Interactive;
 /// <see cref="IMasterKeyProvider.KekId"/> derived identically to the configured providers so a key
 /// entered by hand can decrypt (and re-wrap) rows another provider kind sealed.
 /// </summary>
+[Collection("Process environment")]
 public sealed class LiteralMasterKeyProviderTests
 {
     private static string NewKeyBase64()