builder: work around change in filepath.EvalSymlinks behavior (#4050)

**What type of PR is this?**

> Bug fix

**What does this PR do? Why is it needed?**

filepath.EvalSymlinks works differently on Windows in Go 1.23.0: mount
points and reparse points are no longer treated as symbolic links by
os.Stat. Bazel uses junctions (implemented as reparse points) for
inputs, so this means EvalSymlinks can fail with some kinds of inputs
where it didn't before.

replicateTree calls EvalSymlinks before constructing an input tree for
`GoStdlib`. This seems to be necessary in some cases, though I'm not
altogether sure why. EvalSymlinks fails due to the new behavior though.

This change sets `GODEBUG=winsymlink=0` when invoking the builder binary
to revert to the old behavior. This should not affect the behavior of
user code compiled using the builder. This may not be a permanent
solution, but it should work at least through Go 1.25.

**Which issues(s) does this PR fix?**

Fixes #4049

**Other notes for review**

No test, but verified locally. Simply running `GoStdlib` with Go 1.23.0
will cover this. rules_go currently builds with Go 1.21.8 (no longer a
version supported by Google!).

go/private/context.bzl

diff --git a/go/private/context.bzl b/go/private/context.bzl
index 247c4a3..1eb69a5 100644
--- a/go/private/context.bzl
+++ b/go/private/context.bzl
@@ -503,6 +503,14 @@
         #
         # See https://go.dev/doc/toolchain for more info.
         "GOTOOLCHAIN": "local",
+
+        # NOTE(#4049): Since Go 1.23.0, os.Readlink (and consequently
+        # filepath.EvalSymlinks) stopped treating Windows mount points and
+        # reparse points (junctions) as symbolic links. Bazel uses junctions
+        # when constructing exec roots, and we use filepath.EvalSymlinks in
+        # GoStdlib, so this broke us. Setting GODEBUG=winsymlink=0 restores
+        # the old behavior.
+        "GODEBUG": "winsymlink=0",
     }
 
     # Path mapping can't map the values of environment variables, so we pass GOROOT to the action