build(python): make `NOX_SESSION` optional (#135)

I added this accidentally in #889. `NOX_SESSION` should be passed down if it is set but not marked required.

Source-Author: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>
Source-Date: Tue Jan 19 09:38:04 2021 -0700
Source-Repo: googleapis/synthtool
Source-Sha: ba960d730416fe05c50547e975ce79fcee52c671
Source-Link: https://github.com/googleapis/synthtool/commit/ba960d730416fe05c50547e975ce79fcee52c671
diff --git a/.trampolinerc b/.trampolinerc
index c7d663a..383b6ec 100644
--- a/.trampolinerc
+++ b/.trampolinerc
@@ -18,7 +18,6 @@
 required_envvars+=(
     "STAGING_BUCKET"
     "V2_STAGING_BUCKET"
-    "NOX_SESSION"
 )
 
 # Add env vars which are passed down into the container here.
diff --git a/synth.metadata b/synth.metadata
index d10948c..fd86311 100644
--- a/synth.metadata
+++ b/synth.metadata
@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/python-api-core.git",
-        "sha": "abc3d88466a1e55cf459e6c78a6fb7343a29df6f"
+        "sha": "232dab0ad3ef2cca0edfe707d8f90ca0ea200ba2"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "16ec872dd898d7de6e1822badfac32484b5d9031"
+        "sha": "ba960d730416fe05c50547e975ce79fcee52c671"
       }
     }
   ],