added proper exe parsing for watch, supports spaces in tectonic location

[xinslu]

1 line change to solve #1003.

Added quotes around exe_name to support running watch -x command for users with spaces in their tectonic bin location.

[codecov]

Codecov Report

Merging #1053 (55f4b0c) into master (ff9f29e) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

❗ Current head 55f4b0c differs from pull request most recent head 8bca940. Consider uploading reports for the commit 8bca940 to get more accurate results

@@            Coverage Diff             @@
##           master    #1053      +/-   ##
==========================================
- Coverage   45.83%   45.83%   -0.01%     
==========================================
  Files         171      171              
  Lines       64145    64145              
==========================================
- Hits        29399    29398       -1     
- Misses      34746    34747       +1     

Impacted Files	Coverage Δ
src/bin/tectonic/v2cli.rs	43.20% <0.00%> (ø)

... and 1 file with indirect coverage changes

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