Execution rights on an object can corrupt local cache

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/910)
In GitLab by [[Gitlab user @valentindavid]](https://gitlab.com/valentindavid) on Feb 12, 2019, 17:55

## Summary

[//]: # (Summarize the bug encountered concisely)

Because we use linking of files form cache, when an object require execution rights, the execution rights leak into the local cache. This breaks reproducibility of builds, because a file might have no execution right or have it whether another artifact contains the same object with execution rights is on the local cache or not.

## Steps to reproduce

[//]: # (How one can reproduce the issue - this is very important)

- Have element `a.bst` that creates a file `a`.
- Have element `b.bst` that build-depends on `a.bst`.
- Build `b.bst` and see that file `a` has no execution rights.
- Have element `c.bst` that build-depends on `a.bst`. It installs `a` and `chmod +x` it.
- Build `b.bst` again.

## What is the current bug behavior?

[//]: # (What actually happens)

`a` has execution rights on the second build.

## What is the expected correct behavior?

[//]: # (What you should see instead)

`a` does not have execution rights on the second build.
## Relevant logs and/or screenshots

[//]: # (Paste any relevant logs - please use code blocks ``` to format console output, logs, and code as it's hard to read otherwise. You can also add a Snippet and link it here. Check the markdown giude on Gitlab for further tips)

## Possible fixes

[//]: # (If you can, link to the line of code that might be responsible for the problem)

## Other relevant information

[//]: # (Add the BuildStream version. Substitute below the "x" by "1", "2" or the right version. Add more than one version if necessary)

* BuildStream version affected: /milestone %BuildStream_v1.x

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Nell’issue non sono indicati file o test. Inizia con la sequenza di riproduzione descritta per a.bst, b.bst e c.bst e analizza come il collegamento della cache locale gestisce i permessi di esecuzione. Il lavoro è completato quando la ricompilazione di b.bst produce gli stessi permessi senza esecuzione, indipendentemente dal fatto che c.bst sia stato compilato prima.

Scritto dal modello di indicizzazione a partire dal testo della issue.