Document environment variable JULIA_PKG_PRESERVE_TIERED_INSTALLED (#52362)

This is based on

```julia
julia> using Pkg

help?> Pkg.add
...
  │ Note
  │
  │  To change the default strategy to PRESERVE_TIERED_INSTALLED set the env var
  │  JULIA_PKG_PRESERVE_TIERED_INSTALLED to true.
...
```

I suggest to backport this so that it becomes available in the release
docs of Julia v1.9 and newer.

(cherry picked from commit 2c2ea3a)

Author: ranocha

doc/src/manual/environment-variables.md

@@ -240,6 +240,15 @@ If set to `true`, Pkg operations which use the git protocol will use an external
 
 The accuracy of the package resolver. This should be a positive integer, the default is `1`.
 
+### [`JULIA_PKG_PRESERVE_TIERED_INSTALLED`](@id JULIA_PKG_PRESERVE_TIERED_INSTALLED)
+
+Change the default package installation strategy to `Pkg.PRESERVE_TIERED_INSTALLED`
+to let the package manager try to install versions of packages while keeping as many
+versions of packages already installed as possible.
+
+!!! compat "Julia 1.9"
+    This only affects Julia 1.9 and above.
+
 ## Network transport
 
 ### `JULIA_NO_VERIFY_HOSTS` / `JULIA_SSL_NO_VERIFY_HOSTS` / `JULIA_SSH_NO_VERIFY_HOSTS` / `JULIA_ALWAYS_VERIFY_HOSTS`