show cmd used with --verbose (even on success); fix #783

[timotheecour]

before PR:
nimble install -y -p:-u:release https://github.com/genotrance/nimgen
doesn't show the actual command used and instead shows the un-informative:
Building nimgen/nimgen.exe using c backend
even with --verbose

after PR:
shows the actual cmd used with --verbose, eg:

cmd /Users/timothee/.nimble/bin/nim c --noNimblePath -d:NimblePkgVersion=0.5.1 -u:release -d:release --path:"/Users/timothee/.nimble/pkgs/c2nim-0.9.14"  --path:"/Users/timothee/git_clone/nim/nim-regex/src"  --path:"/Users/timothee/git_clone/nim/nim-unicodedb/src"  --path:"/Users/timothee/.nimble/pkgs/unicodeplus-0.5.1"  --path:"/Users/timothee/git_clone/nim/nim-unicodedb/src"  -o:"/var/folders/ll/dtd6ksrn50n3xkdy4y61tf_80000gn/T/nimble_16626/githubcom_genotrancenimgen/nimgen" /var/folders/ll/dtd6ksrn50n3xkdy4y61tf_80000gn/T/nimble_16626/githubcom_genotrancenimgen/src/nimgen.nim

• which (besides being generally useful) actually shows why -u:release wasn't effective prior to this PR, since -d:release was passed after; I'm also fixing that in this PR so that -p:-u:release is honored

• fix [CI] travis always fails for nim=#hash after the 1st run #783

example

nim c -o:nimble2 src/nimble.nim
./nimble2 install -y https://github.com/genotrance/nimgen
./nimble2 install -y -p:-u:release --verbose https://github.com/genotrance/nimgen
(or ./nimble2 install -y -p:-u:release --verbose https://github.com/genotrance/nimgen to verify the other change I did)

[timotheecour]

PTAL

[dom96]

Tests need to be fixed as well, are they flaky?

[timotheecour]

PTAL

Tests need to be fixed as well, are they flaky?

that's because of #783

I'm also fixing #783 in this PR

[dom96]

Looking good. Thanks!