style(sqllab): fixed button width as label changes

[mistercrunch]

As the label change from RUN to STOP, the row of button shifts
left/right. This fixes the width so that it doesn't jitter.

[codecov-commenter]

Codecov Report

Merging #10404 into master will decrease coverage by 4.97%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #10404      +/-   ##
==========================================
- Coverage   70.41%   65.44%   -4.98%     
==========================================
  Files         604      604              
  Lines       32429    32429              
  Branches     3298     3298              
==========================================
- Hits        22836    21224    -1612     
- Misses       9487    11021    +1534     
- Partials      106      184      +78     

Flag	Coverage Δ
#cypress	?
#javascript	59.33% <100.00%> (+<0.01%)	⬆️
#python	69.75% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...end/src/SqlLab/components/RunQueryActionButton.tsx	69.56% <100.00%> (+1.38%)	⬆️
superset-frontend/src/SqlLab/App.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/explore/App.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/dashboard/App.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/explore/index.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/dashboard/index.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/setup/setupColors.js	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/chart/ChartContainer.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/setup/setupFormatters.js	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/explore/reducers/index.js	0.00% <0.00%> (-100.00%)	⬇️
... and 144 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3fdc141...ed7ff10. Read the comment docs.