expression: fix precision when casting float to string

[tiancaiamao]

What problem does this PR solve?

Fix #9123

What is changed and how it works?

Cast float32(208.867) to float64 will get float64(208.86700439), and
convert the later to string will get a result with wrong precision

Check List

Tests

• Unit test

PTAL @XuHuaiyu @eurekaka

[codecov-io]

Codecov Report

Merging #9137 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #9137      +/-   ##
=========================================
- Coverage   67.21%   67.2%   -0.02%     
=========================================
  Files         371     371              
  Lines       76982   76985       +3     
=========================================
- Hits        51743   51737       -6     
- Misses      20618   20624       +6     
- Partials     4621    4624       +3

Impacted Files	Coverage Δ
expression/builtin_cast.go	80.01% <100%> (+0.05%)	⬆️
ddl/delete_range.go	75.13% <0%> (-4.24%)	⬇️
store/tikv/scan.go	73.94% <0%> (-3.37%)	⬇️
expression/schema.go	94.11% <0%> (-0.85%)	⬇️
executor/join.go	78.38% <0%> (+0.52%)	⬆️
executor/index_lookup_join.go	77.91% <0%> (+0.63%)	⬆️

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[eurekaka]

LGTM

[eurekaka]

@tiancaiamao does this fix need to be cherry-picked to release branches?

[tiancaiamao]

Maybe I should do that @eurekaka

[zz-jason]

LGTM

[zz-jason]

/run-all-tests