chore(ci): fix typo in restrict presubmit samples ITs to only snapshot

Fixes below error in Java repos when googleapis/synthtool#804 was merged:
```
github/java-bigquerystorage/.kokoro/build.sh: line 81: pushd: {SAMPLES_DIR}: No such file or directory
```

Thanks @kolea2 for spotting this in Fusion build logs.

cc @chingor13

Source-Author: Stephanie Wang <stephaniewang526@users.noreply.github.com>
Source-Date: Mon Oct 26 13:44:04 2020 -0400
Source-Repo: googleapis/synthtool
Source-Sha: f68649c5f26bcff6817c6d21e90dac0fc71fef8e
Source-Link: googleapis/synthtool@f68649c

.kokoro/build.sh

@@ -78,7 +78,7 @@ samples)
 
     if [[ -f ${SAMPLES_DIR}/pom.xml ]]
     then
-        pushd {SAMPLES_DIR}
+        pushd ${SAMPLES_DIR}
         mvn -B \
           -Penable-samples \
           -DtrimStackTrace=false \

synth.metadata

@@ -19,7 +19,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "5f6ef0ec5501d33c4667885b37a7685a30d41a76"
+        "sha": "f68649c5f26bcff6817c6d21e90dac0fc71fef8e"
       }
     }
   ],