`choosenim update devel` should be noop if latest version is already built (like D's digger)
`choosenim update devel` should use a cache to avoid rebuilding if it has already built that version.

## current behavior:
```
choosenim update devel
#builds....
choosenim update devel
#builds again...
   Updating devel
Downloading Nim devel from GitHub
[##################################################] 100.0% 0kb/s
 Extracting devel.tar.gz
   Building Nim #devel
   Warning: Building from latest C sources. They may not be compatible with the Nim version you have chosen to install.
Downloading Nim C sources from GitHub
[##################################################] 100.0% 0kb/s
 Extracting master.tar.gz
   Building C sources
   Building koch
   Building Nim
   Building tools (nimble, nimgrep, nimsuggest)
    Updated to #devel
      Info: Version #devel already selected
```

## here's how [digger](https://github.com/CyberShadow/Digger) (D's version of choosenim) does it:
```
digger build "master"
...builds
digger build "master"

Building package digger in /Users/timothee/.dub/packages/digger-3.0.0-alpha-5/digger/
Performing "debug" build using dmd for x86_64.
digger 3.0.0-alpha-5: target for configuration "application" is up to date.
To force a rebuild of up-to-date targets, run again with --force.
Running ../../../.dub/packages/digger-3.0.0-alpha-5/digger/digger build master
digger: Building spec: master
digger: Updating repo...
Fetching origin
digger: Starting at meta repository commit 430ff7ebbe9e6af03d6f52b0ec3fb5dc0ab4a649
digger: Building components dmd, druntime, phobos-includes, phobos, rdmd, tools
digger: needInstalled: dmd-e84c0a07e61a6317c4de74fe072c7387fa411e1a-2550163de58a9523e0fd3f073d355a91
a137c999b2020c324c29f375d9e28a3a48699446 refs/ae-sys-d-cache/dmd-e84c0a07e61a6317c4de74fe072c7387fa411e1a-2550163de58a9523e0fd3f073d355a91
digger: Cache hit!
digger: Installing dmd-e84c0a07e61a6317c4de74fe072c7387fa411e1a-2550163de58a9523e0fd3f073d355a91
digger: needInstalled: druntime-b3dbfc267ab53488a9e285f5315cc7375ff66f4d-d6320d09991e318a195c2fe89d3eba8f
7cc22174d39eafb45252519c4d981e05c468e847 refs/ae-sys-d-cache/druntime-b3dbfc267ab53488a9e285f5315cc7375ff66f4d-d6320d09991e318a195c2fe89d3eba8f
digger: Cache hit!
digger: Installing druntime-b3dbfc267ab53488a9e285f5315cc7375ff66f4d-d6320d09991e318a195c2fe89d3eba8f
digger: needInstalled: phobos-includes-813327d51affb1af8cdd1c7651612e8778109f10-31e8f32f000505d73a97ca3a19c12350
52f85dd62e066fe8a1e54f751be30934a51d8489 refs/ae-sys-d-cache/phobos-includes-813327d51affb1af8cdd1c7651612e8778109f10-31e8f32f000505d73a97ca3a19c12350
digger: Cache hit!
digger: Installing phobos-includes-813327d51affb1af8cdd1c7651612e8778109f10-31e8f32f000505d73a97ca3a19c12350
digger: needInstalled: phobos-813327d51affb1af8cdd1c7651612e8778109f10-bdeaf5cca1a609ed404fca1445341ee6
d906f586c9efa45ef038bc775a2fae7a580ae01c refs/ae-sys-d-cache/phobos-813327d51affb1af8cdd1c7651612e8778109f10-bdeaf5cca1a609ed404fca1445341ee6
digger: Cache hit!
digger: Installing phobos-813327d51affb1af8cdd1c7651612e8778109f10-bdeaf5cca1a609ed404fca1445341ee6
digger: needInstalled: rdmd-9dc0d2e3963a24630f59786b035e498348c9e69d-ab663ab9d026d8591afc27134da7db44
3fcbfc1d1520f9779d3d623f79d5ba7d41f0bee9 refs/ae-sys-d-cache/rdmd-9dc0d2e3963a24630f59786b035e498348c9e69d-ab663ab9d026d8591afc27134da7db44
digger: Cache hit!
digger: Installing rdmd-9dc0d2e3963a24630f59786b035e498348c9e69d-ab663ab9d026d8591afc27134da7db44
digger: needInstalled: tools-9dc0d2e3963a24630f59786b035e498348c9e69d-352635cb7e901824082c1e4ca24809d2
cbf4f78cc79f149992e0b1d5755ed968145f6ed8 refs/ae-sys-d-cache/tools-9dc0d2e3963a24630f59786b035e498348c9e69d-352635cb7e901824082c1e4ca24809d2
digger: Cache hit!
digger: Installing tools-9dc0d2e3963a24630f59786b035e498348c9e69d-352635cb7e901824082c1e4ca24809d2
digger: Moving...
digger: Build successful.
```

## note:
this is related to https://github.com/dom96/choosenim/issues/12 but I couldn't find the above suggestion in that issue, so doesn't look like a dup
