README.vendor: add note about GOROOT, recommend fresh go

Using a different build of Go (specifically, a different GOROOT) to
maintain the vendor directory doesn't always reproduce the same results.
This can result in unknowingly creating a vendor directory that isn't
able to build Go.

Add a note to README.vendor to point this out. Specifically, mention
that a mismatched GOROOT is an issue, and recommend using a fresh build
of Go to maintain the vendor directory.

Updates #69235

Change-Id: Id80c7607bf28bd76e43e1fdc672811c50f2bffb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/616815
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Sam Thanawalla <[email protected]>
Reviewed-by: Sam Thanawalla <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: dagood

src/README.vendor

@@ -33,6 +33,10 @@ Before updating vendor directories, ensure that module mode is enabled.
 Make sure that GO111MODULE is not set in the environment, or that it is
 set to 'on' or 'auto', and if you use a go.work file, set GOWORK=off.
 
+Also, ensure that 'go env GOROOT' shows the root of this Go source
+tree. Otherwise, the results are undefined. It's recommended to build
+Go from source and use that 'go' binary to update its source tree.
+
 Requirements may be added, updated, and removed with 'go get'.
 The vendor directory may be updated with 'go mod vendor'.
 A typical sequence might be: