Updated workflow to only run one job per PR

[4everTheOne]

At the moment, if a contributor opens a PR and is constantly pushing changes to the branch multiple actions will be created for that PR, making the build process slow for everyone.

This PR suggests a change in this behavior, by only building the latest commit. If a build is in progress and the author submits a new change, the current build will be aborted and a new build will start with all the changes.

[4everTheOne]

An example of the build being cancelled can be seen at: https://github.com/javaparser/javaparser/actions/runs/3369330764
The previous action was cancelled due to a new commit being publish: https://github.com/javaparser/javaparser/actions/runs/3369332925

(This was trigger by updating this branch with the master)

[codecov]

Codecov Report

Merging #3744 (a4fd053) into master (106f730) will increase coverage by 0.008%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##              master     #3744       +/-   ##
===============================================
+ Coverage     57.305%   57.313%   +0.008%     
  Complexity      2671      2671               
===============================================
  Files            635       635               
  Lines          33566     33566               
  Branches        5790      5790               
===============================================
+ Hits           19235     19238        +3     
+ Misses         12277     12275        -2     
+ Partials        2054      2053        -1     

Flag	Coverage Δ
AlsoSlowTests	57.313% <ø> (+0.008%)	⬆️
javaparser-core	52.460% <ø> (+0.011%)	⬆️
javaparser-symbol-solver	36.730% <ø> (+0.008%)	⬆️
jdk-10	57.309% <ø> (?)
jdk-11	57.309% <ø> (?)
jdk-12	57.306% <ø> (?)
jdk-13	57.309% <ø> (+4.865%)	⬆️
jdk-14	57.303% <ø> (+0.014%)	⬆️
jdk-15	57.309% <ø> (+0.008%)	⬆️
jdk-16	57.309% <ø> (+0.008%)	⬆️
jdk-8	57.311% <ø> (+0.017%)	⬆️
jdk-9	57.309% <ø> (+0.011%)	⬆️
macos-latest	57.302% <ø> (ø)
ubuntu-latest	57.299% <ø> (+0.004%)	⬆️
windows-latest	57.293% <ø> (?)

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

Impacted Files	Coverage Δ
...ava/com/github/javaparser/utils/LineSeparator.java	81.355% <0.000%> (+1.694%)	⬆️
...urces/com/github/javaparser/TokenMgrException.java	74.358% <0.000%> (+5.128%)	⬆️

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