fix: fix paths in barretenberg bootstrap.sh script (#1662)

In #1258, we added a
`cd` into the build directory but didn't `cd` back out again. This means
that all the paths for the rest of the script are incorrect.


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- [ ] If the pull request requires a cryptography review (e.g.
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circuits/cpp/barretenberg/cpp/bootstrap.sh

@@ -54,6 +54,7 @@ cd ./build
 cmake --build . --parallel --target grumpkin_srs_gen
 ./bin/grumpkin_srs_gen 8192
 echo "Generated Grumpkin SRS successfully"
+cd ../
 
 # Install wasi-sdk.
 ./scripts/install-wasi-sdk.sh