Guix Cuirass - Continuous Integration - Crash Error

["Reza Alizadeh Majd" <r.majd@pantherx.org>]

Hi Guix, 

I want to setup a CI build server using _Cuirass_ to build a series of custom 
packages for Guix. but I receive following error and cuirass-service crashes, 
could anyone help me on this? 

```
2019-06-03T18:50:59 fatal: uncaught exception 'git-error' in 'build' fiber!
2019-06-03T18:50:59 exception arguments: (#<<git-error> code: -14 message: "the index is locked; this might be due to a concurrent or crashed process" class: 10>)
In ice-9/boot-9.scm:
    829:9  9 (catch _ _ #<procedure 2da2560 at cuirass/utils.scm:17?> ?)
    705:2  8 (call-with-prompt _ _ #<procedure 2da2240 at ice-9/eval?>)
    705:2  7 (call-with-prompt _ _ #<procedure 2dab940 at ice-9/eval?>)
In ice-9/eval.scm:
    619:8  6 (_ #(#(#<directory (guile-user) 236e140> #<variable?> ?)))
In srfi/srfi-1.scm:
    640:9  5 (for-each #<procedure process (spec)> (((#:name . #) ?)))
In ice-9/boot-9.scm:
   751:25  4 (dispatch-exception 0 git-error (#<<git-error> code: -?>))
In cuirass/utils.scm:
    181:8  3 (_ _ #<<git-error> code: -14 message: "the index is loc?>)
In ice-9/boot-9.scm:
    829:9  2 (catch #t #<procedure 2dd44e0 at cuirass/utils.scm:181?> ?)
In cuirass/utils.scm:
   182:22  1 (_)
In unknown file:
           0 (make-stack #t)
ERROR: In procedure make-stack:
Git error: the index is locked; this might be due to a concurrent or crashed process
```


In case that it might be helpful, here is my `%cuirass-specs` definition: 
```
(define %cuirass-specs
   #~(list '((#:name . "my-manifest")
             (#:load-path-inputs . ("guix"))
             (#:package-path-inputs . ("custom-packages"))
             (#:proc-input . "guix")
             (#:proc-file . "build-aux/cuirass/gnu-system.scm")
             (#:proc . cuirass-jobs)
             (#:proc-args . ((subset . "manifests")
                             (systems . "x86_64-linux")
                             (manifests . (("config" . "manifest.scm")))))
             (#:inputs . (((#:name . "pantherx")
                           (#:url . "https://user:pass@git.pantherx.org/development/guix-pantherx.git")
                           (#:load-path . ".")
                           (#:branch . "master")
                           (#:no-compile? #t)))))))
. . .

(service cuirass-service-type
			    (cuirass-configuration
			      (interval 10)
			      (use-substitutes? #t)
			      (port 8082)
			      (host "0.0.0.0")
			      (specifications %cuirass-specs)))
```


I also added `manifest.scm` file beside my system configuration file with following
content:
```
(use-modules (px packages accounts))
(packages->manifest
   (list px-accounts))
```


---
Best, 
Reza