Describe the bug
When the versionSpec input is configured to 3.0.0, the first run caches the cli, and the subsequent looks for the cli on PATH but fails.

To Reproduce
Steps to reproduce the behavior:

1. Configure a pulumi config task as below:

    - task: Pulumi@1
      displayName: Pulumi config
      env:
        AZURE_STORAGE_ACCOUNT: $(pulumiStateStorageAccount)
        AZURE_STORAGE_KEY: $(pulumiStateStorageAccountKey)
        PULUMI_CONFIG_PASSPHRASE: $(pulumiSecret)
        PULUMI_SELF_MANAGED_STATE_LOCKING: 1
      inputs:
        azureSubscription: '$(azureSubscription)'
        command: 'config'
        loginArgs: 'azblob://$(pulumiStateContainer)'
        stack: 'my-stack'
        versionSpec: '3.0.0'
        createStack: true

2. Run the task. The first execution downloads the tool and caches it:

Downloading: https://get.pulumi.com/releases/sdk/pulumi-v3.0.0-linux-x64.tar.gz
Extracting archive
/usr/bin/tar xC /azp/agent/_work/_temp/2f278ff1-a72c-49f7-9278-65e3e388xxxx -f /azp/agent/_work/_temp/44fa164e-c998-4f35-bbf3-10d64b65xxxx

Prepending PATH environment variable with directory: /azp/agent/_work/_temp/2f278ff1-a72c-49f7-9278-65e3e388xxxx/pulumi
Caching tool: pulumi 3.0.0 x64
/azp/agent/_work/_temp/2f278ff1-a72c-49f7-9278-65e3e388xxxx/pulumi/pulumi version
v3.0.0

3. Run the task again. This time the task looks for the tool in cache and finds it. It prepends a folder to path. But the task then runs a which command that is not able to find the cli on path and gives this output:

Found tool in cache: pulumi 3.0.0 x64
Prepending PATH environment variable with directory: /azp/agent/_work/_tool/pulumi/3.0.0/x64
##[error]Pulumi CLI does not seem to be installed in your environment.

4. Removing versionSpec fixes it, as it doesn't look for the cached tool.

Expected behavior
Adding the cache path to $PATH should have made the cli available to the task.