build(python): make NOX_SESSION optional (#197)

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: googleapis/synthtool@ba960d7

.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.

synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/python-bigtable.git",
-        "sha": "604dde3aa4aa292d7ec9598917ade7acf1c93f5f"
+        "sha": "227482f8fd5ea500b3677348ee3fa25e486a2163"
       }
     },
     {
@@ -19,14 +19,14 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "16ec872dd898d7de6e1822badfac32484b5d9031"
+        "sha": "ba960d730416fe05c50547e975ce79fcee52c671"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "16ec872dd898d7de6e1822badfac32484b5d9031"
+        "sha": "ba960d730416fe05c50547e975ce79fcee52c671"
       }
     }
   ],