bootstrap: Prefer 'python3'

'python' is more likely to be an alias.

Change-Id: I5d61b623e9a9e38b83af762c79cffa3dd49b75ff
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/41620
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Joe Ethier <jethier@google.com>
diff --git a/bootstrap.sh b/bootstrap.sh
index ad44a66..589b3a2 100644
--- a/bootstrap.sh
+++ b/bootstrap.sh
@@ -15,7 +15,7 @@
 # This script must be tested on bash, zsh, and dash.
 
 _bootstrap_abspath () {
-  $(command -v python || command -v python3 || command -v python2) -c "import os.path; print(os.path.abspath('$@'))"
+  $(command -v python3 || command -v python2 || command -v python) -c "import os.path; print(os.path.abspath('$@'))"
 }
 
 # Users are not expected to set PW_CHECKOUT_ROOT, it's only used because it