tool/go: update to correct toolchain as needed

Ensure that the target revision is loaded from this repository, not from
the working directory.
Update go to the target revision if the marker file does not match the
target.

Updates #501

Signed-off-by: James Tucker <james@tailscale.com>

tool/go

@@ -8,6 +8,8 @@ if [[ "${CI:-}" == "true" && "${NOBASHDEBUG:-}" != "true" ]]; then
     set -x
 fi
 
+tsandroid=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )/.." &> /dev/null && pwd )
+
 # Allow TOOLCHAINDIR to be overridden, as a special case for the fdroid build
 if [[ -z "${TOOLCHAINDIR}" ]]; then
     toolchain="$HOME/.cache/tailscale-go"
@@ -29,10 +31,13 @@ if [[ -z "${TOOLCHAINDIR}" ]]; then
             rm -rf "$toolchain" "$toolchain.extracted"
         fi
     fi
-    if [[ ! -d "$toolchain" ]]; then
-        mkdir -p "$HOME/.cache"
 
-        read -r REV <go.toolchain.rev
+    REV="$(<${tsandroid}/go.toolchain.rev)"
+    EREV=""
+    [[ -f ${toolchain}.extracted ]] && EREV="$(<${toolchain}.extracted)"
+
+    if [[ ! -d "$toolchain" || "$EREV" != "$REV" ]]; then
+        mkdir -p "$HOME/.cache"
 
         case "$REV" in
         /*)