test: set dummy value for NUGET_PACKAGES

[nikpivkin]

Description

Tests for nuget may fail if the packages used in the tests are located on the developer's machine in the ~/.nuget/packages or NUGET_PACKAGES directory, since the analyser uses this directory to search for licenses:

--- FAIL: TestRepository (8.14s)
    --- FAIL: TestRepository/nuget (0.01s)
        /Users/nikita/projects/trivy/integration/integration_test.go:304: 
...
            	            	-     Licenses: ([]string) <nil>,
            	            	+     Licenses: ([]string) (len=1) {
            	            	+      (string) (len=3) "MIT"
            	            	+     },
...

This PR sets a dummy value for the NUGET_PACKAGES environment variable.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[knqyf263]

LGTM. Left a small comment.

[knqyf263]

What if using t.TempDir?

Suggested change

	t.Setenv("NUGET_PACKAGES", "/tmp/fake_nuget_packages")
	t.Setenv("NUGET_PACKAGES", t.TempDir())

[nikpivkin]

Makes sense. Fixed 26fb4f9