Improve/Fix Paket system installation docs -> No local build required

Description

System-wide Paket install fails with error FS0192: internal error: null: convTypeRefAux .

mono 6.0.0.313
msbuild 16.0.0.0
fsharpc 10.2.3 for F# 4.5

Repro steps

Please see https://nextjournal.com/mpd/paketmono-install for my minimalized attempt.

apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb https://download.mono-project.com/repo/ubuntu stable-bionic main" | \
  tee /etc/apt/sources.list.d/mono-official-stable.list
  
apt-get -qq update
DEBIAN_FRONTEND=noninteractive apt-get install \
  mono-devel mono-complete libunwind-dev libssl1.0-dev \
  fsharp

git clone https://github.com/fsprojects/Paket
cd Paket
./build.sh

Expected behavior

A working system-wide Paket installation is created.

Actual behavior

FsiEvaluationException:

Error: 
	
	error FS0192: internal error: null: convTypeRefAux
	

Output: [Loading /Paket/paket-files/build/fsharp/FAKE/modules/Octokit/Octokit.fsx
	 Loading /Paket/build.fsx]
	

Input: /Paket/build.fsx
\Arguments: 
  C:\fsi.exe
  --define:MONO

Exception: Yaaf.FSharp.Scripting.FsiEvaluationException: Error while compiling or executing fsharp snippet. ---> System.Exception: Operation failed. The error text has been printed in the error stream. To return the corresponding FSharpErrorInfo use the EvalInteractionNonThrowing, EvalScriptNonThrowing or EvalExpressionNonThrowing
  at Microsoft.FSharp.Compiler.Interactive.Shell+FsiEvaluationSession.commitResult[a,b] (Microsoft.FSharp.Core.FSharpChoice`2[T1,T2] res) [0x00030] in <58ebd1c7ddab8ea7a7450383c7d1eb58>:0 
  at Microsoft.FSharp.Compiler.Interactive.Shell+FsiEvaluationSession.EvalScript (System.String filePath) [0x00017] in <58ebd1c7ddab8ea7a7450383c7d1eb58>:0 
  at Yaaf.FSharp.Scripting.Helper+evalScript@1303.Invoke (System.String arg00) [0x00000] in <5c90a2e0ccf1c534a7450383e0a2905c>:0 
  at Yaaf.FSharp.Scripting.Helper+save_@1276-2[a].Invoke (Microsoft.FSharp.Core.Unit unitVar0) [0x00000] in <5c90a2e0ccf1c534a7450383e0a2905c>:0 
  at Yaaf.FSharp.Scripting.Helper.consoleCapture[a] (System.IO.TextWriter out, System.IO.TextWriter err, Microsoft.FSharp.Core.FSharpFunc`2[T,TResult] f) [0x00019] in <5c90a2e0ccf1c534a7450383e0a2905c>:0 
  at Yaaf.FSharp.Scripting.Helper.redirectOut@1247[a] (System.Boolean preventStdOut, Yaaf.FSharp.Scripting.Helper+OutStreamHelper out, Yaaf.FSharp.Scripting.Helper+OutStreamHelper err, Microsoft.FSharp.Core.FSharpFunc`2[T,TResult] f) [0x0006c] in <5c90a2e0ccf1c534a7450383e0a2905c>:0 
  at Yaaf.FSharp.Scripting.Helper+save_@1275-1[a].Invoke (System.String text) [0x0002a] in <5c90a2e0ccf1c534a7450383e0a2905c>:0 
   --- End of inner exception stack trace ---
  at Yaaf.FSharp.Scripting.Helper+save_@1275-1[a].Invoke (System.String text) [0x0009c] in <5c90a2e0ccf1c534a7450383e0a2905c>:0 
  at Yaaf.FSharp.Scripting.Helper+session@1306.Yaaf-FSharp-Scripting-IFsiSession-EvalScriptWithOutput (System.String path) [0x00000] in <5c90a2e0ccf1c534a7450383e0a2905c>:0 
  at Fake.FSIHelper.runScriptUncached (System.Boolean useCache, System.String scriptPath, System.Collections.Generic.IEnumerable`1[T] fsiOptions, System.Boolean printDetails, Fake.FSIHelper+CacheInfo cacheInfo, System.IO.TextWriter out, System.IO.TextWriter err) [0x00195] in <5c90a2e0ccf1c534a7450383e0a2905c>:0 
error FS0192: internal error: null: convTypeRefAux

CACHING ERROR - please open a issue on FAKE and /cc @matthid

Error: System.NotSupportedException: Type 'FSI_0005.Build' was not completed.
  at System.Reflection.Emit.ModuleBuilder.Save () [0x00048] in <81342d83acda4c5590ec19c7afdf26b9>:0 
  at System.Reflection.Emit.AssemblyBuilder.Save (System.String assemblyFileName, System.Reflection.PortableExecutableKinds portableExecutableKind, System.Reflection.ImageFileMachine imageFileMachine) [0x0022b] in <81342d83acda4c5590ec19c7afdf26b9>:0 
  at System.Reflection.Emit.AssemblyBuilder.Save (System.String assemblyFileName) [0x00000] in <81342d83acda4c5590ec19c7afdf26b9>:0 
  at Fake.FSIHelper.handleCaching[a] (System.Boolean printDetails, Yaaf.FSharp.Scripting.IFsiSession session, a fsiErrorOutput, System.IO.DirectoryInfo cacheDir, Fake.FSIHelper+CacheInfo cacheInfo) [0x00044] in <5c90a2e0ccf1c534a7450383e0a2905c>:0 

I'm completely new to this whole ecosystem, so I may be confused as to what I'm supposed to be installing, but this seemed like it should work.

Start by reproducing the Ubuntu/Mono setup from the issue and inspect build.sh and build.fsx, including the FAKE/Octokit script path shown in the error. Compare the documented installation flow with the minimal reproduction; done means the system-wide Paket installation works without requiring a local build, or the limitation and supported steps are clearly documented.