Fixed permissions for local build

[SerheyDolgushev]

This PR is about changing permissions for the built bootstrap file.

In the last released tag (https://github.com/softprops/serverless-rust/releases/tag/v0.3.8) integer 755 value is used

serverless-rust/index.js

Line 152 in c17cb6c

755

Which is incorrect.

In the latest master octal 0o755 value is used: https://github.com/softprops/serverless-rust/blob/master/index.js#L172

But this also causes some issues. Ways to reproduce:

1. Clone https://github.com/SerheyDolgushev/serverless-rust-example
2. Run hello rust lambda function:

   npx serverless invoke local -f hello

3. Try to unzip target/lambda/release/lambda.zip using unzip command:

   unzip target/lambda/release/lambda.zip 
   Archive:  target/lambda/release/lambda.zip
      skipping: bootstrap               volume label

4. It is still possible to unzip target/lambda/release/lambda.zip just by double-clicking on it. But extracted bootstrap will have wrong permissions in this case (-r--r--r-- so execute permission is missing):

   ls -l target/lambda/release/bootstrap 
   -r--r--r--  1 sdolgushev  72107521  6765776 Mar 25 09:32 target/lambda/release/bootstrap

The issue is fixed with this PR.