Fix readme for wrong use of fish subshell in string

[kirrmann]

The scripts fails and outputs an error cd: The directory '(mktemp -d)' does not exist. This error appears because () in strings aren't generally recognized as subshells in fish. To fix this I set a variable which we use to cd to. There seems to be no way to quote subshell commands in fish correctly.

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[codecov-io]

Codecov Report

Merging #215 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #215   +/-   ##
=======================================
  Coverage   54.35%   54.35%           
=======================================
  Files          13       13           
  Lines         701      701           
=======================================
  Hits          381      381           
  Misses        269      269           
  Partials       51       51

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[kirrmann]

registered to CLA

[kirrmann]

recheck CLA

[corneliusweig]

/check-cla

[kirrmann]

recheck CLA

[ahmetb]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahmetb, kirrmann

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