Specifying environment variables for intellisense project loader

## Issue Description ##
Unable to find mechanism for providing environment variables to Omnisharp for intellisense.

My `.csproj` files sometimes contain paths to local projects, using environment variables. Eg
```

```

I have been unable to find any mechanism to provide these environment variables to Omnisharp so I get proper intellisense.

Since Omnisharp does not have these variables I get error messages in the omnisharp log, eg
```
Microsoft.Build.Exceptions.InvalidProjectFileException: The imported project "/tasks/CsTask.targets" was not found
```

When I build from command line I have the environment variable configured and can build correctly. As well as it is possible to provide environment variables for `.vscode/tasks.json` and `.vscode/launch.json` when running debug/build commands.

I see omnisharp has support for `.env` file, could this not also be used for general omnisharp loading and intellisense?

Or is there some other mechanism available? I know I can launch VS Code from a terminal with the environment configured, but I would rather be able to launch it "normally" and have some kind of local configuration with these variables that Omnisharp can consume.

## Environment information ##

**VSCode version**: 1.49.0
**C# Extension**: 1.23.2

Mono Information
OmniSharp using built-in mono
Dotnet Information
.NET Core SDK (reflecting any global.json):
Version: 3.1.402
Commit: 9b5de826fd

Runtime Environment:
OS Name: Mac OS X
OS Version: 10.14
OS Platform: Darwin
RID: osx.10.14-x64
Base Path: /usr/local/share/dotnet/sdk/3.1.402/

Host (useful for support):
Version: 3.1.8
Commit: 9c1330dedd

.NET Core SDKs installed:
3.1.402 [/usr/local/share/dotnet/sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.App 3.1.8 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 3.1.8 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download

Visual Studio Code Extensions

|Extension|Author|Version|
|---|---|---|
|auto-close-tag|formulahendry|0.5.8|
|auto-comment-blocks|kevinkyang|1.0.1|
|auto-rename-tag|formulahendry|0.1.4|
|auto-using|Fudge|0.7.15|
|better-toml|bungcip|0.3.2|
|blazorsnippets|ScottSauber|1.3.0|
|bracket-pair-colorizer-2|CoenraadS|0.2.0|
|brick|koliyo|0.1.0|
|camelcasenavigation|maptz|1.1.3|
|change-case|wmaurer|1.0.0|
|cmake|twxs|0.0.17|
|cmake-tools|ms-vscode|1.4.1|
|code-settings-sync|Shan|3.4.3|
|cpptools|ms-vscode|0.30.0-insiders5|
|csharp|ms-dotnettools|1.23.2|
|debugger-for-chrome|msjsdiag|4.12.10|
|dhall-lang|panaeon|0.0.4|
|dotenv|mikestead|1.0.1|
|ginfuru-vscode-jekyll-syntax|ginfuru|0.1.1|
|gitlens|eamodio|10.2.2|
|go|golang|0.16.2|
|gradle-language|naco-siren|0.2.3|
|hjson|laktak|0.3.2|
|insert-unicode|brunnerh|0.8.0|
|jinjahtml|samuelcolvin|0.15.0|
|jsonnet|heptio|0.1.0|
|jumpprotobuf|ldcf4|0.0.3|
|kotlin|fwcd|0.2.18|
|language-julia|julialang|1.0.7|
|language-prometheus|caarlos0|0.1.2|
|language-stylus|sysoev|1.11.0|
|lit-html|bierner|1.11.1|
|lsp-inspector-webview|octref|0.0.2|
|markdown-all-in-one|yzhang|3.3.0|
|material-theme|zhuangtongfa|3.8.7|
|mono-debug|ms-vscode|0.15.8|
|npm-intellisense|christian-kohler|1.3.1|
|prettify-json|mohsen1|0.0.3|
|project-manager|alefragnani|11.3.0|
|python|ms-python|2020.8.108011|
|QML|bbenoist|1.0.0|
|remote-ssh|ms-vscode-remote|0.55.0|
|remote-ssh-edit|ms-vscode-remote|0.55.0|
|restructuredtext|lextudio|129.0.0|
|rust|rust-lang|0.7.8|
|scriban|xoofx|1.1.0|
|shader|slevesque|1.1.4|
|swig-tpl|zhangciwu|0.0.6|
|terraform|hashicorp|2.2.3|
|vetur|octref|0.27.3|
|vscode-docker|ms-azuretools|1.5.0|
|vscode-eslint|dbaeumer|2.1.8|
|vscode-fileutils|sleistner|3.0.1|
|vscode-graphql|Prisma|0.3.2|
|vscode-icons|vscode-icons-team|10.2.0|
|vscode-jekyll-snippets|ginfuru|0.9.3|
|vscode-jest|Orta|3.2.0|
|vscode-journal|pajoma|0.10.0|
|vscode-markdownlint|DavidAnson|0.36.3|
|vscode-new-file|dkundel|4.0.2|
|vscode-nginx|shanoor|0.6.0|
|vscode-npm-script|eg2|0.3.13|
|vscode-nuget-package-manager|jmrog|1.1.6|
|vscode-open-in-github|ziyasal|1.3.6|
|vscode-open-in-sublime-merge|morrislaptop|0.6.0|
|vscode-paste-and-indent|Rubymaniac|0.0.8|
|vscode-proto3|zxh404|0.4.2|
|vscode-test-explorer|hbenl|2.19.1|
|vscode-theme-onedark|akamud|2.2.2|
|vscode-typescript-tslint-plugin|ms-vscode|1.2.3|
|vscode-yaml|redhat|0.10.1|
|vscode-yarn|gamunu|1.7.1|
|vscodeintellicode|VisualStudioExptTeam|1.2.10|
|vsliveshare|ms-vsliveshare|1.0.2740|
|vsnotes|patricklee|0.7.1|
|vue-snippets|hollowtree|0.1.12|;