Use absolute file path for local provider paths

[EronWright]

A simple fix for the LocalProviderPath option, the path should be made absolute in the resultant Pulumi.yaml configuration block.

Repro: given a test case that overrides the local provider path:

test := pulumitest.NewPulumiTest(t, "helm-release-unknowns", opttest.LocalProviderPath("kubernetes", "../../../bin"))

The program fails with:

    /Users/eronwright/Pulumi/pulumi-kubernetes/tests/sdk/nodejs/helm_test.go:33: failed to installing packages and plugins: exit status 255
        error: parsing plugin options for 'kubernetes': no folder at path '../../../bin'

I believe this is because Pulumi attempts to resolve the provided path using the workspace directory. As is done with go.mod replacements, the test framework must resolve the path more eagerly.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (8c45fd2) 33.45% compared to head (f751d46) 36.78%.
Report is 5 commits behind head on main.

Files	Patch %	Lines
pulumitest/newStack.go	57.14%	2 Missing and 1 partial ⚠️
pulumitest/install.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #66      +/-   ##
==========================================
+ Coverage   33.45%   36.78%   +3.33%     
==========================================
  Files          42       42              
  Lines        2571     2634      +63     
==========================================
+ Hits          860      969     +109     
+ Misses       1606     1561      -45     
+ Partials      105      104       -1     

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