Use portable shebang in .envrc template for NixOS compatibility

[abs]

Summary

NixOS does not have /bin/bash, so generated .envrc doesn't work. This changes /bin/bash to portable /usr/bin/env bash.

How was it tested?

Ran go build -o ./devbox-test ./cmd/devbox && ./devbox-test generate direnv && direnv reload && cat .envrc

Community Contribution License

All community contributions in this pull request are licensed to the project maintainers under the terms of the Apache 2 License.

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