tool/gocross, pull-toolchain.sh: support a "next" Go toolchain

When TS_GO_NEXT=1 is set, update/use the
go.toolchain.next.{branch,rev} files instead.

This lets us do test deploys of Go release candidates on some
backends, without affecting all backends.

Updates tailscale/corp#36382

Change-Id: I00dbde87b219b720be5ea142325c4711f101a364
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tool/gocross/gocross-wrapper.sh

@@ -4,7 +4,7 @@
 #
 # gocross-wrapper.sh is a wrapper that can be aliased to 'go', which
 # transparently runs the version of github.com/tailscale/go as specified repo's
-# go.toolchain.rev file.
+# go.toolchain.rev file (or go.toolchain.next.rev if TS_GO_NEXT=1).
 #
 # It also conditionally (if TS_USE_GOCROSS=1) builds gocross and uses it as a go
 # wrapper to inject certain go flags.
@@ -21,6 +21,12 @@ if [[ "${OSTYPE:-}" == "cygwin" || "${OSTYPE:-}" == "msys" ]]; then
     exit
 fi
 
+if [[ "${TS_GO_NEXT:-}" == "1" ]]; then
+    go_toolchain_rev_file="go.toolchain.next.rev"
+else
+    go_toolchain_rev_file="go.toolchain.rev"
+fi
+
 # Locate a bootstrap toolchain and (re)build gocross if necessary. We run all of
 # this in a subshell because posix shell semantics make it very easy to
 # accidentally mutate the input environment that will get passed to gocross at
@@ -45,7 +51,7 @@ cd "$repo_root"
 # https://github.com/tailscale/go release artifact to download.
 toolchain=""
 
-read -r REV <go.toolchain.rev
+read -r REV <"$go_toolchain_rev_file"
 case "$REV" in
 /*)
     toolchain="$REV"
@@ -148,7 +154,7 @@ unset GOROOT
 # gocross is opt-in as of 2025-06-16. See tailscale/corp#26717
 # and comment above in this file.
 if [ "${TS_USE_GOCROSS:-}" != "1" ]; then
-    read -r REV <"${repo_root}/go.toolchain.rev"
+    read -r REV <"${repo_root}/$go_toolchain_rev_file"
     case "$REV" in
     /*)
         toolchain="$REV"