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/ProcessEnvironmentCollection.cs

@@ -0,0 +1,10 @@
+namespace ZB.MOM.WW.Secrets.Tests;
+
+/// <summary>
+/// Serializes every test class that mutates process environment variables. setenv/getenv operate
+/// on the single process-wide environ, which is not safe under concurrent mutation on glibc —
+/// unique variable names do not remove the race, only serialization does. Classes that never
+/// touch the environment stay fully parallel.
+/// </summary>
+[CollectionDefinition("Process environment", DisableParallelization = true)]
+public sealed class ProcessEnvironmentCollection;