Race Condition for multiple tools spawned via direnv exec

[ryansch]

Describe the bug
I'm running a lot of automated processes via editor tooling; language servers, linters, etc. I'm seeing a lot of extra errors due to race conditions when trying to set up the devenv environment.

Since a lot of these tools are spawned at the same time, it seems that they race to create the various symlinks required by devenv.

Examples:

• ln: .devenv/profile: File exists
• ln: .devenv/profile/a4jvj862kmjmlgmqc4f17166n3qhp5b6-devenv-profile: Permission denied

To reproduce
I've created an example rails project with a lot of the tooling we use in our real repos:
https://github.com/detaso/rails-tooling

Version
devenv: 0.6.2

[domenkozar]

This is fixed in #745