cipd: delay reading version files

This info is only used when bootstrapping for the first time,
so move these vars to that function to avoid wasted effort.

Bug: 1469696
Change-Id: I21b97b2429cf2dac6242e2e86aa7668b09ed47c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4752396
Reviewed-by: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>

cipd

@@ -94,7 +94,6 @@ CIPD_BACKEND="https://chrome-infra-packages.appspot.com"
 VERSION_FILE="${MYPATH}/cipd_client_version"
 
 CLIENT="${MYPATH}/.cipd_client"
-VERSION=$(<"${VERSION_FILE}")
 PLATFORM="${OS}-${ARCH}"
 
 # A value in .cipd_client_platform overrides the "guessed" platform.
@@ -103,7 +102,6 @@ if [ -f "${PLATFORM_OVERRIDE_FILE}" ]; then
   PLATFORM=$(<"${PLATFORM_OVERRIDE_FILE}")
 fi
 
-URL="${CIPD_BACKEND}/client?platform=${PLATFORM}&version=${VERSION}"
 USER_AGENT="depot_tools/$(git -C ${MYPATH} rev-parse HEAD 2>/dev/null || echo "???")"
 
 
@@ -170,6 +168,9 @@ function clean_bootstrap() {
     exit 1
   fi
 
+  local VERSION=$(<"${VERSION_FILE}")
+  local URL="${CIPD_BACKEND}/client?platform=${PLATFORM}&version=${VERSION}"
+
   # Download the client into a temporary file, check its hash, then move it into
   # the final location.
   #