Autofill SOURCE_DATE_EPOCH and SOURCE_GIT_HASH during vendoring (#464)

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This is a small fix to autofill correct defaults for SOURCE_DATE_EPOCH
and SOURCE_GIT_HASH when running `just vendor`. See
https://github.com/pop-os/cosmic-epoch/issues/3571 for further context.

justfile

@@ -92,15 +92,15 @@ vendor:
     echo 'directory = "vendor"' >> .cargo/config.toml
     echo >> .cargo/config.toml
     echo '[env]' >> .cargo/config.toml
-    if [ -n "${SOURCE_DATE_EPOCH}" ]
-    then
+    if [ -z "${SOURCE_DATE_EPOCH}" ]; then
+        SOURCE_DATE_EPOCH=$(git log -1 --format='%ct')
+    fi
+    if [ -z "${SOURCE_GIT_HASH}"]; then
+        SOURCE_GIT_HASH=$(git rev-parse HEAD)
+    fi
     source_date="$(date -d "@${SOURCE_DATE_EPOCH}" "+%Y-%m-%d")"
     echo "VERGEN_GIT_COMMIT_DATE = \"${source_date}\"" >> .cargo/config.toml
-    fi
-    if [ -n "${SOURCE_GIT_HASH}" ]
-    then
     echo "VERGEN_GIT_SHA = \"${SOURCE_GIT_HASH}\"" >> .cargo/config.toml
-    fi
     tar pcf vendor.tar .cargo vendor
     rm -rf .cargo vendor